If your favorite extension gets disabled by upgrading to the newer version some may downgrade to older Firefox version that extension worked before, some may sacrifice incompatible extensions for the sake of Firefox and stay on latest Firefox.
The case with mine where I want to use Firefox 9 and at the same time can’t loose FDM as I don’t want to download files with other download managers as I used to FDM since some time for now.
Manually making incompatible add-on compatible with Firefox
Though you can change Firefox version number to version that FDM worked before manually by searching for install.rdf file of extension in your profile, but it’s a bit tedious (I didn’t succeeded in that though worked in enabling Google Toolbar though).
Another way is type about:config in address bar and proceed by agreeing to the warning, right-click and select New>Boolean and type preference name to extensions.checkcompatibility.9.0 and set its value to False.
Making add-ons compatible through add-ons
Nightly Tester tools add-on which is specifically designed for users in testing nightly builds, this add-on forces the addon compatibility and it does this by automatically creating extensions.checkcompatibility.x.0 preference name (where x will be your browser version) and by setting its value to false.
Add-on Compatibility Reporter is another add-on specifically designed for alpha and beta users to test and report to Mozilla whether their add-ons working or having some issues in alpha and beta versions of Firefox they’re using.
This add-on disables add-on compatibility check and hence unless you disable or remove incompatible addon it won’t be disabled by Firefox. Finally add-on compatibility checker allowed me to enable and use FDM as usual again.