Since some older version addon does not work on updated version of Firefox, I have been setting my Firefox to bypass the version checking on addons…

And seems current version (3.6) has change how the setting is set, Googling like 30 secs found the answer but I think it is good to keep a note on it.

>about:config in firefox.
>new boolean extensions.checkCompatibility.3.6 to false
>quit and reload firefox

REF:
MozillaZine – Updating add-ons – Read from line “Note: Starting with Gecko 1.9.2 applications (e.g., Firefox 3.6)”