Every time there's a new version of Firefox, a large number of extensions stop working, and while there's a Firefox hack you can use to force compatibility, you can also just manually hack a single extension.
Over at the How-To Geek site (my home away from Lifehacker), writer Asian Angel has a detailed guide to hacking any Firefox extension to be compatible with any version of Firefox — and the only tool you need is Notepad.
To perform this hack for yourself, you can rename any downloaded Firefox extension to .ZIP, unzip it, and then change the compatibility numbers in the install.rdf file. Once you've done so, zip the contents up again, rename to .XPI, and then you can simply drag the file into the Firefox add-ons window to install it. Hit the link for the full walk-through guide, or check out our guide to making your extensions work with Firefox 3.6 and above.