You are correct in that XP is not compatible with IE 9 and never will be to my knowledge.
I would recommend upgrading FF to the latest version to see if this fixes your problem, not to mention the upgrade provides add some security features.
I also use ABP in FF, and don't have an issue with XP-H SP3 or with XP-Pro SP3, so I don't think it is an ABP problem.
If after upgrading FF, you still have a problem, you may want to try a different theme with the Avast forum and find one that you like. However when you sign out, it will go back to the default, which will give you the problem again. I, too, also have to scroll sideways to see everything since I don't have a mega-screen.
But I'm fine when I'm signed into the forum.