UPDATE: For Firefox-4, enter about:config through location bar, agree to the silly warning, and create a new Boolean key named extensions.checkCompatibility.4.0 and set its value to 4. If the key already exists just change its value.
Sometimes you just can't live without an Addon which has not been updated or upgraded for the recent versions of Firefox. Although it is always recommended to use the latest version of an Addon for security and performance reasons, your productivity can suffer if you suddenly find your Firefox updated and one of your favourite Addons missing.
There's a trick you can use to install an old/discontinued Firefox Addon on a version that is not accepting the Addon.
DISCLAIMER: I assume no responsibility whatsover for this advice. Your Firefox/Computer may become unusable for all I now!
Step by Step Guide
- Install Firefox Addon called Nightly Tester Tools
- Restart Firefox
- Find your Old Addon on Mozilla and Add to Firefox.
- As soon as it is detected that the Addon is not compatible, Nightly Tester Tools will offer a popup window with the button Force Install. Click that Button.
- Restart Firefox
- Enjoy your incompatible Addon. You can now uninstall Nightly Tester Tools if you want. Your Forced Installed incompatible Addon will continue to work.