If you've ever used a pre-release version of Firefox (be it Alpha or Beta), you'll probably have noticed that your extensions/add-ons aren't compatible.
I've been able to work around this in the past, by modifying the extension itself, which is a pain, but will often do the trick if you absolutely can't live without the extension.
On the other hand, you may not need the extension that badly, but rather, you just want to be able to start Firefox without it complaining about extension compatibility. An easy way to tell Firefox to "pipe down" comes courtesy of Asa Dotzler:
about:configin the Firefox's address bar
- Right-click and choose "New > Boolean"
- When prompted for the Preference name, enter
- Chose the value
- Restart your browser
The nice thing about this is that it will speed up the start-time for pre-release versions of Firefox, since the time it takes for extension compatibility checks will be eliminated.