You should update to the latest release, overall if you are using a 64 bits OS. Lot’s of 64 bit specific stuff has been fixed since then. Besides lots of other bugs.
Not really. The gtk support in fvwm adds a dependency on gtk1, and is used only to show gtk menus in fvwm or something like that (I really never used it and always compiled fvwm without it).
Your problem might lie elsewhere, and maybe it doesn’t even lie on fvwm itself (firefox acts funky everywhere), but try fvwm 2.5.25, and maybe you will have some luck with it.