The answer to the first question is: not a lot. Previous updates to Firefox have changed the look and feel of the open source web browser, but we had to check that we were using Firefox 5, so similar does it appear to Firefox 4. Our tests of Firefox 5 suggest that there are, however, key improvements in this iteration that should make you swiftly upgrade.
Firefox 5: Speed
It's quicker and less resource hungry. When first we started browsing with Firefox 5, it felt more nippy than its predecessor: not earth-shatteringly so, but enough to notice. More importantly, after growing increasingly frustrated with Mozilla Firefox 4's propensity to slow down and fall over, this release seems lighter on its feet (and less heavy on the memory). Could Firefox 5 be the Windows 7 to Firefox 4's Windows Vista? Objective testing, he say: 'yes'.
This is important. In recent months I've more or less switched to Google Chrome, even for using the PC Advisor Content Management System - which is written for Firefox. Quite simply, I was losing too much work when Firefox chewed up a ton of memory and fell over. Any improvement here helps.
Firefox 5: Security
So Firefox 5 is quicker and smaller, it's also - according to Mozilla - safer. Because Do Not Track will now feature in Firefox for Android, Mozilla says its is the first web browser to support Do Not Track on multiple platforms. Version 5 of the desktop product certainly puts the feature front and centre. Go to Options, and it's right there at the top: tick the checkbox and participating websites will know not to store your data. How useful that is I leave for you to decide, but it speaks to Mozilla's desire for Firefox to be seen as the browser of choice for the security conscious (and the paranoid).
Similarly, Mozilla says that in this version of Firefox WebGL content can no longer load cross-domain textures. WebGL is an API by which websites can display 3D imagery. It utilises the end user's graphics hardware to render complex images.
Firefox 5: The Open Web
Mundane, but more fundamentally important: Firefox 5 brings improved standards support for HTML5, XHR, MathML, SMIL and canvas. Firefox 5 for Android enjoys added IPv6 support. So right now Firefox 5 is the best browser for open Web standards. It also adds support for CSS animations, a Firefox Add-on SDK for Windows, Mac and Linux and the Firefox Add-on Builder Beta, which provides a web-based build environment. So if you are a web developer or Firefox add-on maker, Firefox 5 is categorically a good thing.
Firefox 5: And The Rest...
Mozilla is claiming 'more than 1,000 improvements and performance enhancements' in Firefox 5. You may not notice them, but improvements and enhancements sound good, right? But you may find that add-ons you use in Firefox 4 are not yet compatible. Extensions for Firefox are almost all third-party apps, so developers have a bit of a scramble to catch up when Mozilla updates the browser. In our case, four of our add-ons were incompatible, but Firefox does at least periodically check for updates, and install the apps without you having to do anything other than approve the change.
See also: Group test: what's the best web browser?