have you (like me and many others) upgraded to Firefox version 4 to find most of your add-ons, that you loved, won't work under FF4.
Well I have the solution for you:
1. Type about:config in your Firefox browser address bar and agree to the warning message.
2. Now right click inside the browser, choose New –> Boolean, type the preference name as extensions.checkCompatibility.4.0, click OK and set it to False.
Your old add-ons will work with Firefox 4 and you will also be able to download and install any of the other incompatible add-ons from the Internet.
I now have my EXIF viewer and Histogram Viewer Add-Ons back. ENJOY
Note: you may have to google to find some of the add-ons as they often will not appear via the Firefox Add-ons menu. For example, inside FF4 when I went to add-ons and searched for histogram it did not find my viewer, but googling 'firefox histogram add-on' did!