I use MozBackup regularly. That isn't the point. For an add-on to pass the Firefox "checking to see if your add-ons are compatible" (which WebRep did) it should work properly without the user having to jump through flaming hoops. It doesn't. Therefore, I will simply wait until it DOES work. It is currently in a 'disabled' state and FF will occasionally check to see if it is updated. When it is, then I'll re-enable it. Currently, the WebRep add-on will NOT work with FF4.0 and needs to be checked out with the developer (Avast!). This is the main reason I posted here - not to find a 'work-around'.
I do appreciate that a full removal and reinstall will 'solve' the problem, but, once again, that's not the point. Besides, I have already upgraded all my FF versions from 3.x to 4.0 and your solution would entail procedures that, if they didn't work properly, might just make a simple irritant turn into a real mess.