You can do this by adding a new preference value.
1. Point your firefox browser at the URL to “about:config“.
2. Right-click on the preferences list to bring up the contextual menu. You should see an option that says “New.” Select that, and choose “Boolean.”
3. When it asks you for the preference name, type “extensions.checkCompatibility” (without the quotes). You have to enter the name exactly. For the value, choose “false.”
4. Restart Firefox. Firefox should give you a warning that version checking is disabled. You can double-check under “Tools:Add-Ons” to make sure everything is activated once again.
Just in case anything bad happens, there’s always a backup plan. Just tell Firefox to “Reset Prefrences”, restart and you will have extension compatibility turned back on again.