Every good web developer has two responsibilities: check the web application's behavior in multiple browsers and submit found browser's bugs and incompatibilities to the browser's vendor. Having so much browser versions even from a single browser vendor complicates developer's life. Luckily there is a simple way to test your web application in various Firefox versions. If you already have some Firefox version installed on your computer all you have to do is to download a new Firefox version (the newest version from trunk can be found here) extract it in some folder and run it using ProfileManager option (after you have closed all Firefox instances go to command prompt and do something like this: D:\firefoxes\3.2a1pre>firefox.exe -ProfileManager).