Whenever a new version of Firefox is released, there might be many add-ons which are not compatible and Firefox internally checks them and if found in compatible, it disables them. Although there is a parameter called
extensions.checkCompatibility which can be set to false to disable add-on compatibility check, this has been now customized and you need to add this parameter for each different versions of Firefox. For example, if you are using Firefox 4.0 beta then the code is
extensions.checkCompatibility.4.0b and if you are using final version, its
Disable Add-on Compatibility Checks is a nice add-on for Firefox which reinstates the extensions.checkCompatibility preference without respect to the current application version. So irrespective of the Firefox version, you can easily disable the add-on compatibility checks.
Once installed, the add-on does not require any restart, you can start using it right away and the compatibility check is disabled.
If you do not want to install the add-on, you can try out the trick for disabling add-on compatibility check using our tutorial.