Quixin, you've done this before, right?
Yes, Its very simple, Install Mr Tech's Local Install 4.0. You can download it
here. Once its installed, restart your Firefox obviously. Now you have the ability to override the maxVersion check. For extensions that are already installed (Labeled “Disabled - not compatible with Firefox 1.5") you can right click and choose "Make Compatible".
For installing new extensions, Attempt to install the extensions you want. On the "Software Installation" window, you will see a box at the bottom that says "Override 'maxVersion' compatibility checking". Put a check mark in that box and you should be able to install.