There’s a better way than enabling indiscriminately all of your Firefox extensions at once.
If you had an indispensable extension which got disabled as soon as you upgraded to Firefox 3.5, there’s still hope,
as long as the extension’s code is compatible (which you’ll find out once you’re done re-enabling the extension).
First go to your Mozilla profile folder and locate the extension you’re
interested in. This will be in one of the folders in your Mozilla profile folder (e.g.
C:\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\
094a7r2w.default\extensions\{89736E8E-4B14-4042-8C75-AD00B6BD3900}).
Copy this folder somewhere else, then go to [Tools], [Add-ons] and Uninstall the extension. Close Firefox, copy
the folder back to its original location and before restarting Firefox, open install.rdf and change the extension’s
em:maxVersion to 3.5.* (for more information in locating this refer to
Mozilla Developer Centre). Now start
Firefox again. You should be able to see your extension again. Chances are that it will still be functional.