Use the "MR Tech Toolkit" extension to make this old extension work. Get it here:
MR Tech Toolkit has two features that can help in this situation.
1. If you're just installing Tab X for the first time, MR Tech Toolkit has a feature that will allow you to override version checking during installation.
2. If you already have Tab X installed, but it stopped working when you installed a Firefox update, then you can go to the "Add-ons" window, right-click on Tab X, and select "Make compatible".
Use caution with either of these methods. Tab X isn't guaranteed to work with this version of Firefox, obviously, so bad things could happen. Also, use caution with MR Tech Toolkit, it enables you to make changes to Firefox that could cause you trouble.