Many 'old' addons CAN work in FF 4, except that their configuration file explicitly states that its "maxVersion" (the highest level of compatibility) is less than 4. Something like "3.6*" or "3.*". The instructions below tell you how to extract this file from the .xpi package, edit it in a text editor, and reinsert it into the package. Then you simply hit Ctrl-O to Open a file, then select your newly-modified .xpi file, and that's it.
I tried it with CuteMenus and it works great! My only advice is that you change the version NOT to 4.0*, but to 4.* (otherwise, if this addon isn't updated before version 4.1 of FF comes out, it will stop working when you upgrade).
If there are other non-updated addons you want to try with this, just remember that some plugins still won't work, because deep down they really are incompatible; but most ARE compatible except for the small technical detail that this fix corrects.