Disable Firefox Extension Compatibility Check to use Old Add-ons

     Most of old Add-ons don't work on new Firefox such as Firefox 3.6, Firefox 3.7, 4.0, even Firefox 3.5. Although you can edit add-ons files (*.xpi) to make it support newer Firefox version, but this way is complicated and waste time.
     And the best way is directly disable compatibility check of Firefox. With this way, you can install any add-ons to any Firefox version without warning about version support, fast and easily. But note that, you can install not mean you can use it normally, maybe get some error or even add-on can't run (rarely). I've used this way since Firefox 3.0 beta and everything are alright.                                                                                                                                       

Now, let's do it. Follow these steps:

     1/ Open Firefox, type about:config at address bar and Enter. Press I'll be careful, I promise! to continue.





     2/ Now, you will see Firefox config page, right click on blank area and choose New > Boolean, at New Boolean Value dialog, type this name: extensions.checkCompatibility and then OK.





     3/ In next dialog, you must choose False. If you choose True, Extenstion Compatibility is still working. Last, restart you Firefox and start install any add-ons you want.

NOTE:

     - This tip works fine on all version of Firefox 3.x and include Firefox 4.x.
     - After did this step, when open Add-ons Manager, you will see yellow bar talks that Add-on Compatibility is disable. You can click Enable button to enable it (should not). Furthermore, you can see a warning icon at some add-ons icon with note "... not compatible with Firefox...". Just ignore it.

Video Tutorials: