If you've updated Firefox to the latest version and found that your favorite addon isn't compatible with it yet, you can regain access to it by downloading a second, older version of Firefox for use specifically with that addon. This page lists the most recent version if Firefox before your current version, with a link to its download.
On a Mac, when you drag it to your Applications folder, be sure to click "Keep both versions." The older version of Firefox will be called "Firefox 2" and all of your addons are already installed on it.
You can still use the latest version for your main browsing, but if you need the feature that addon gave you, you can jump over to the archived older version.