The new Firefox 3.5.7 and Firefox 3.0.17 also included a patch for a bug that Mozilla's programmers introduced in mid-December, when they last issued a security update.
Mozilla regularly offers the newer Firefox 3.5 to users running the older Firefox 3.0 with prominent billboard-like dialogue boxes as part of a process the company calls 'major update'.
The uptake on the several major updates offered since last summer, however, has been well under Mozilla's estimates.
After rooting around Firefox, Mozilla's developers discovered that a coding issue was putting only a small notification on the screen rather than the full upgrade offer.
"What's happening is that users who do not leave their browser open for 12 hours (i.e.: they close it once a day) will never see the major update dialogue, only a little notification slider," said Mike Beltzner, the director of Firefox, in a thread on Bugzilla, Mozilla's bug- and change-tracking database.
Earlier in the thread, Beltzner called the omission "simply shocking".
Mozilla attributed the lower-than-expected uptake of the major upgrade offer to the bug, which Beltzner characterised as a 'regression' flaw that had not been caught by internal code reviews or the company's QA process.
But he was optimistic that the change would boost the number of users migrating from Firefox 3.0 to Firefox 3.5.
"Added bonus: we're about to goose our Firefox 3.5 numbers!" Beltzner said.
As of the end of December, Firefox 3.0 had a 6.9 percent share of the browser market, according to web metrics vendor Net Applications, while Firefox 3.5 had a share of 16.3 percent.
Migration from the old to the new had been brisk in July, August and September 2009, but fell sharply during the next three months.
Getting users to switch to Firefox 3.5 is important because Firefox 3.0 will be retired later this month from security patch support.
Mozilla also eliminated a bug it introduced in Firefox 3.5.6 and Firefox 3.0.16 , the mid-December updates.
The fix in yesterday's editions was to NTLM (NT LAN Manager) authentication, which had been patched last month to eliminate a vulnerability rated "high" by Mozilla.
Unfortunately, the patch prevented users from using corporate proxy servers to access the Internet.
NTLM is a Microsoft-made authentication protocol used with the company's SMB (Server Message Block) file-and print-sharing protocol in Windows.
The release of Firefox 3.5.7 and 3.0.17 came just days before Mozilla plans to roll out the release candidate (RC) of Firefox 3.6, an upgrade it's been working on since last spring.
Last month, Mozilla stuck a fifth beta of Firefox 3.6 into its schedule, which in turn pushed the final ship date of the update into this year.
Yesterday, Beltzner said Mozilla had started crafting RC builds; in meeting notes published today, Mozilla said it was aiming for a Friday release of Firefox 3.6 RC.
At the same time, Mozilla programmers are discussing whether to tackle a quick-turnaround update to Firefox dubbed 'Lorentz' that would add several features to the browser by March.
The most notable change would be to separate external plug-ins, such as Adobe's Flash, from Firefox's other processes so that if the plug-in crashed, the browser wouldn't follow suit.
Developers have been arguing about Lorentz's merits since late December on a Mozilla mailing list .
It's unclear if Lorentz was synonymous with Firefox 3.7, another minor upgrade that had been slated for a March ship date, or would replace that edition. Beltzner declined to discuss Lorentz today, citing his workload as Firefox 3.6's release nears.
Firefox 3.5.7 and 3.0.17 can be downloaded for Windows, Mac OS X and Linux from the Mozilla site. Current Firefox users can instead call up the browsers' update tools, or wait for automatic update notifications to appear in the next 48 hours.