Having Firefox 5 final version released with some minor improvements from Firefox 4 but the only problem you may face is some add-ons worked in Firefox 4 may not compatible with Firefox 5 and if you don’t want to sacrifice your favorite add-ons for Firefox 5 here is the guide to ( help you to get back to Firefox 4 without loosing your settings, bookmarks, add-ons which is called downgrading) downgrade from Firefox 5 to Firefox 4 or earlier versions .
Downgrading from Firefox 5 to Firefox 4.0.1 or older versions
1. Visit Mozilla FTP Server in your browser.
2. Open Firefox 4.0.1 folder >Win32 folder >en-US
3. Download and install Firefox setup 4.0.1 exe file
You’ll be automatically downgraded to Firefox 4.0.1 and your favorite add-ons, bookmarks and settings will remain untouched and disabled add-ons will be enabled.
Note : You need to disable update checking for Firefox to stay on Firefox 4 or Firefox 3.6.x by opening Tools> Options>Advanced>Update uncheck for “Firefox” for automatically check for updates to” option.
Update 21/06/11 : Post updated.