To stop everyone from simply ignoring the compatibility checks and using problematic add-ons that haven't been updated in a while, Mozilla removed the ability to turn off extension compatibility checking entirely and now are forcing users to add a separate about:config entry for the specific version they are using.
So instead of adding a new entry called extensions.checkCompatibility, you'll need to add the Firefox version to the preference name, followed by "a" or "b" if you are using alpha or beta versions, respectively. For the current Firefox 3.6 Beta release, you will need to add a boolean key with the following name, and a value of false:
If you aren't already an about:config expert or just want some hand-holding, hit the link for the full walk-through to make your extensions compatible. Readers will probably note that you can also use the Nightly Tester Tools extension, but for those of us that would rather not install another bulky extension, this tweak works perfectly.
How To Force Extension Compatibility with Firefox 3.6+ [How-To Geek]