So a quick and ugly workaround I found was to let the build proceed as it likes (e.g. with both gtk2 and gtk3), then `%exclude /usr/lib64/newmoon/gtk2/libmozgtk.so` in the spec's `%files` section. It appears to work fine (and no crashes so far -- yahoo!) but I'm not sure I haven't e.g. broken themin...
I know this thread is getting a bit old, but it's the most recent I could find which mentions gtk3 builds. I'm currently attempting to build (for a OpenSuSE-based micro distro I maintain on the OBS) Palemoon against GTK3. The reasons for this basically come down to gtk2 breakage which has crept in, ...