The most important reason users doesn’t try the latest specially the beta version of Mozilla Firefox is fear of loosing of an important add-on. Because of these non-compatible add-on user is left with only one option that is to downgrade the browser which is also a compromise with security, as newer version have several security and stability updates.
So if you want to use add-ons which are not compatible with the latest version of Mozilla Firefox, you have to install another add-on called Nightly Tester Tool which allows you to use non compatible add-on.
Steps for making Non-Compatible Add-On Work under Latest / Beta Version
Download and Install Nightly Tester Tools
Download Mozilla Firefox 3.6 Beta 4
Open extension dialog box from Tools/Add-ons and right click on extension which is disabled, and choose Override Compatibility.
Click on Force Install Button to install the disabled Add-on.
After force installation restart the browser and enjoy your favorite Add-on