For all of those who just downloaded Firefox 1.5 and can’t get your extensions to work. This is a simple modification of Firefox’s configuration to trick the extensions to work. No need to manually edit the extensions to get them to work. Currently all the extensions I use work.

1. At the location bar, enter: about:config. This will show you a list of Firefox internal preferences.
2. Right-click on the list, select New > String
   Enter “app.extensions.version” (without quotes) for the preference name.
3. Then, enter “1.0” (without quotes) as the value for app.extensions.version.
4. Restart Firefox 1.5, then enable those disabled Firefox extensions.
5. Restart Firefox 1.5 again to active the extensions. Done.

Personally I think this is the best solution because you can always go back and delete the string later rather then fiddling with the extensions.