Option #1. Mozilla earlier created the Compatibility Reporter add-on for its users to test and report the compatibility of add-ons with various Firefox releases.
The interesting part about this add-on is that it will automatically configure your Firefox to install and use add-ons that are otherwise incompatible with your release of Firefox.
Option #2. If you don’t like installing one more extension to your Firefox build just for enabling older add-ons, here’s a workaround.
Type about:config in your browser address bar and agree to the warning message. Now right click inside the browser, choose New –> Boolean, type the preference name as extensions.checkCompatibility.4.0b and set it to False.
That’s it. Your old add-ons will work with Firefox 4 and you will also be able to download and install any of the other incompatible add-ons from the Internet. If you interested in the technical details, check this mozillaZine entry.
Option #3. This is the geekiest option of them all but one that doesn’t require you to change anything inside Firefox.
You can download the extension (the .xpi file) to your computer, unzip it and change the value of em:maxVersion under install.rdf file to 4.0.* – now pack the extension again and drag-n-drop to your Firefox browser. Please refer to mozillaZine for detailed step-by-step instructions.
Also see: Digital Inspiration Add-ons Pack