1. Type "about:config" in your browser. (Without quotes of course.)
2. Right-click on preferences, which brings up a menu. There should be an
option that says "New." Click on it, and then choose "Boolean."
3. It will ask for the preference name, so type in
"extensions.checkCompatibility" (again, without the quotes). You must
type it in exactly as shown here. When it asks for a value, be sure to
choose "false."
4. Now you need to restart FF. FF will likely inform you that
"version checking is disabled", which is exactly what you want. Confirm
changes by going to "Tools:Add-Ons".
Voila.