Sometimes when Firefox gets updated, it says that one or more of your add-ons are incompatible and it disables them. If updated to that add-on are not available then you are pretty much stuck without using that add-on. The same problem is seen when you try to install an incompatible add-on. But you can force Firefox to install and use incompatible add-ons. Here is how :
- Start Mozilla Firefox.
- Type about:config in the location bar and press Enter.
- Firefox will show a warning. Click on the button labeled I’ll be careful I promise! to proceed.
- Right-click anywhere on the preferences list and select New → Boolean
- Type in extensions.checkCompatibility.[version] in the Preference name, where [version] is the Firefox version. For example, I am using Firefox 3.5.8, then I would type extensions.checkCompatibility.3.5. If you have Firefox version 4.0.1, then append 4.0 in place of [version] etc.
- Choose a value false when asked.
- Restart the browser for the changes to take effect.
Note that installing incompatible add-ons may crash Firefox. If that happens, then you have to start Firefox in Safe mode and then change this setting to true. After that you can restart Firefox back in the normal mode.