Firefox is one of open source's biggest success stories. Not only is it a hit with mainstream users, it's also popular with developers, who've created a number of spinoffs, derivatives, and alternative builds from its code base.

Alternative builds of Firefox exist mainly to take advantage of processor instructions that aren't used in the official Mozilla versions. Mozilla codes Firefox so that it will run on the broadest possible number of machines. But that breadth of compatibility comes with a performance penalty. The instruction sets used by alternative builds -- and unused in Mozilla's official edition -- provide a speed boost.

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Under-the-hood changes in private builds might also include custom features (such as Atelier's portable mode), but just as often they involve disabling standard Firefox features (such as the removal of accessibility extensions and parental controls in Pale Moon) -- again, for the sake of better performance and stability.

Another common reason for alternative builds is to create native 64-bit editions of Firefox, which can address larger memory spaces and in theory perform better than their 32-bit counterparts. It's safe to assume any PC shipped today is running a 64-bit version of Windows, but many apps remain available only in 32-bit editions for compatibility. Firefox is one such app, in large part because of the significant number of 32-bit Windows systems still in use.

I've long been curious about the state of third-party Firefox builds. Here I've surveyed the most prevalent third-party builds to see whether or not they're worth the trouble of swapping in to replace stock editions of Firefox. My criteria for picking these particular builds? First, they're actively maintained. Second, they're optimized for 64-bit systems or, at the very least, just plain optimized for faster performance. And third, they're available in a Windows edition, important because Windows constitutes the biggest installation base for Firefox.

In every case but one, I tested builds that used the 10.0 branch of Firefox. The lone exception, Atelier, is based on the 9.2 version of Firefox. I also ran the browsers against a set of common browser benchmarks:

Peacekeeper, a general JavaScript and HTML5 test suite

SunSpider, WebKit's JavaScript test suite, which tests core language behavior only (not the DOM or browser APIs)

BrowserMark, a JavaScript and HTML rendering benchmark nominally for mobile browsers but also useful for desktop versions

I also tried out the SPDY download-speed test, to compare the browsers that had it enabled (all builds based on Firefox 10 and later) with those that didn't.

The test system was a quad-core Intel Q6600, 2.4GHz, with 4GB of RAM running Windows 7 Ultimate.

Firefox remixes: Pale Moon

Pale Moon is currently one of the most popular and widely discussed Firefox rebuilds, and I suspect that's in big part because it's remarkably well maintained as a project. Unlike many other alternative builds, Pale Moon enjoys a properly built site devoted to it (instead of just a hastily assembled download repository), with solid documentation, a support forum, and proper explanations of everything in well-written English. Such professionalism alone makes Pale Moon all the more appealing, even if it's still delivering only the 9.2 version of Firefox.

Pale Moon exists in two basic version branches: the 3.6 version of Firefox's core code and the 9.2 version (32-bit or 64-bit). The main difference is processor compatibility; the 3.6 branch is for people stuck with PIII and Athlon XP processors, while 9.2 is for those using "a 7th or later generation CPU with SSE2 support," as Pale Moon's own documentation puts it. If you're wondering why Pale Moon's creators are using version 9.2 and not 10 or later, it's because they are planning to skip version 10 entirely.

When launched for the first time, Pale Moon brings up two tabs. First is a "Welcome to Pale Moon" tab that's reminiscent of Firefox's own first-run page, with general remarks about the browser and its relationship to Firefox. The other is a custom start page with a Google search bar and links to many popular destinations: Gmail, Twitter, the Firefox add-on repository, and so on. None of this is obligatory; I was able to replace it with my own custom start page easily enough.

Some custom builds of Firefox use the existing Firefox profile directory, which means they run the risk of trashing said profile if anything goes wrong. Pale Moon keeps its own profile directory separate to avoid such problems, and the migration tool is painless enough to use. To that end, Pale Moon's maintainers offer a profile migration tool so that Firefox users can copy their user profile into Pale Moon.

Firefox users won't find much outwardly missing or changed in Pale Moon, but a few stock Firefox features have been disabled: the crash reporter, the parental controls, and the accessibility functions. This last one may be a bone in the throat for people who depend on such items. But in many other ways that matter, Pale Moon behaves just like Firefox. Add-ons, for instance -- when you browse for new ones, you're taken directly to Mozilla's add-on repository, and add-ons behave exactly as they ought to. If you want to run Pale Moon in any language other than English, you'll need to do a little under-the-hood hackery -- nothing too major, fortunately -- in addition to downloading the appropriate language pack.

Firefox remixes: Waterfox

Billed as the fastest 64-bit variant of Firefox, Waterfox is certainly snappy, although not dramatically faster than the 32-bit editions out there. Its most recent version is 10.0.2, keeping pace with Firefox itself.

As implied, Waterfox is 64-bit only -- there is no 32-bit version, so a 64-bit processor and OS are both mandatory. You'll need to install 64-bit editions of common plug-ins too, but Waterfox's maintainers have thoughtfully supplied direct-download links for each one from their downloads page. I was able to set up each of them -- Java, Flash, and Silverlight -- with no trouble.

If you already use Firefox, the first thing you're likely to notice about Waterfox is how it inherits your default Firefox profile. Launch Waterfox and you'll see your bookmarks and history already present. This sounds like a good idea, but I worry that running both Waterfox and Firefox off the same profile might risk corruption of profile data.

To that end, I booted Waterfox with the -profilemanager switch and created a separate account. The maintainers of Waterfox deserve some credit for bringing attention to this issue, as there's a big warning on their site to this effect: "If you uninstall Waterfox make sure you don't have the remove personal data box ticked!" They do not, however, provide instructions for creating a separate profile, although that's not hard to unearth.

It's the lack of documentation, compared to what Pale Moon provides, that makes Waterfox a lesser choice. Its website is skimpy, with only the most basic information about how the product was built. Actual support for the product isn't even provided on the site itself, but rather through a thread in the forums. Then again, at least some degree of support is provided, which is more than I expected.

Firefox remixes: Lawlietfox

The product of a Taiwanese programmer who goes by the sobriquet "Lawliet" (a reference to "Death Note," most likely), Lawlietfox comes in both 32- and 64-bit editions. All of the builds -- going back to version 8 -- are entirely install-it-yourself affairs. They're only available as 7-Zip format archives, with no Microsoft Installer (MSI) packages or other setup functions. On the plus side, it means all you need to do to get Lawlietfox up and running is unpack the archive into a folder somewhere and run Firefox.exe.

Be advised that, like Waterfox, Lawlietfox will use the most recent Firefox profile it finds on your machine. There is no warning about this -- not in the program itself and none that I could discern on the project's site. Run Firefox.exe -profilemanager the first time you use Lawlietfox and set up a separate profile to avoid trashing anything.

Nothing is outwardly all that different about Lawlietfox, other than the name of the product, so don't worry about the standard Firefox look being tinkered with here. Launching the program on a clean profile brings up the Firefox Community Edition start page (about:home) with the usual Google search. Most everything else works as you'd expect; add-ons and plug-ins all behave well, and the add-on updater functions correctly.

Sadly, documentation is all but nonexistent, and feedback in the issue tracker is in Chinese only. You're very much on your own, which is par for the course when using any unofficial build of Firefox but doubly so here and trebly so if you download Lawlietfox's pre-release version 11 build. That edition contains support -- added by Mozilla, not Lawliet -- for the SPDY protocol, which theoretically reduces load times and latency for Web pages.

Firefox remixes: Tete's Atelier

Japanese programmer Tete creates his own private builds of Firefox named the Atelier builds. Some of the patches he's created for Firefox are used in other builds (such as Lawlietfox), and his work is presented with a bit more documentation and thoroughness than most custom builds. For instance, Tete documents obvious known bugs, a big plus.

As with most of the other custom builds, Atelier comes with no installer. Unpack the program into a folder, set up a separate user profile (here again, there's no warning in the documentation about possibly trashing an existing user profile), and fire it up. Note there are no 64-bit editions of Atelier, not even of the bleeding-edge version 11 beta branch. And unlike the other third-party builds reviewed here, all Firefox branding remains in place: the Firefox name, the Firefox logo, and so on. I'm not sure if this is permitted under Mozilla's branding and licensing terms.

That said, Atelier's 32-bit edition of 10.0.2 was so snappy that a 64-bit edition almost seems superfluous. The benchmark tests show Atelier builds providing the most noticeable performance improvement over stock Firefox -- borne out by using the browser instead of just running benchmarks on it.

One concession Atelier makes to users who don't want to set up a separate profile is a custom-made portable mode. Enable this (it requires some .ini file hacking) and the program can use any Firefox profile directory you specify, rather than the default one created by Firefox in the Windows user profile folder. This also makes Atelier a little easier than other browsers to hand-integrate into a solution like the PortableApps launcher.

The documentation for Atelier is a little more complete than most of the other custom builds I've seen, although at the same time it highlights that many more ways a custom build can be less dependable than a stock edition. Some add-ons, for instance, don't work unless you manually hack the user-agent string. Localizing the browser requires additional hackery on top of applying the language pack -- and so on.

Are third-party builds worth considering? They do provide a modest degree of additional speed over stock Firefox builds, but not the night-and-day-difference you might expect. While they may be interesting from a programmer's point of view, they don't make a strong case for Firefox performance being dramatically improved by outsiders changing a few compilation switches.

The biggest performance boost seemed to come from the Atelier builds, with the Lawlietfox and Pale Moon builds running slightly behind. Waterfox's 64-bit build didn't seem to get much of a performance gain merely by dint of being 64-bit, which implies that other optimizations are needed to make Firefox shine in its 64-bit incarnation.

Third-party Firefox builds are a combination of mixed bag and moving target. Aside from the work required to stay on top of Firefox's own constant evolution, each custom build tends to become its own animal -- and an exercise in trade-offs. For example, though Pale Moon is well documented, one of the costs of its relative stability is that it's a few versions behind Firefox.

Mozilla's accelerated pace of revisions for Firefox is one reason third-party builds are becoming a tougher sell than they used to be. An increased rate of releases automatically makes it harder for people to keep up with their own builds, as they have to compile, test, and release much more frequently.

In the long run, I suspect custom Firefox builds created solely for the sake of processor optimization will disappear, especially when Firefox rolls out its own native 64-bit build. (In fact, 64-bit Firefox is already available in the most recent nightly build of version 11.) But for now, there's still a culture of roll-your-own Firefoxers keeping the faith.