The next time you upgrade Firefox—which will be to version 3.1—you don't want to find out that your favorite extension isn't yet compatible with the new version, and Mozilla's going out of their way to make sure that doesn't happen. They've published an eye-popping list of Firefox add-ons which make up 95% of known add-on usage, and show what level of compatibility each extension offers. This report will update as Firefox 3.1 nears release; in the meantime, it's very interesting to browse the list, which is ordered by usage frequency to compare it to your favorite extensions.
Add-on Compatibility Report [Firefox Add-ons]
