This appears to be a problem with the gcc 4.3 in current openSUSE Factory repository.

gcc -v is:
Using built-in specs.
Target: i586-suse-linux
Configured with: ../configure --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --libexecdir=/usr/lib --enable-languages=c,c++,objc,fortran,obj-c++,java,ada --enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.3.0 --enable-ssp --disable-libssp --with-bugurl=http://bugs.opensuse.org/ --with-pkgversion='SUSE Linux' --disable-libgcj --with-slibdir=/lib --with-system-zlib --enable-__cxa_atexit --enable-libstdcxx-allocator=new --disable-libstdcxx-pch --program-suffix=-4.3 --enable-version-specific-runtime-libs --without-system-libunwind --with-cpu=generic --build=i586-suse-linux
Thread model: posix
gcc version 4.3.0 20071109 (experimental) [trunk revision 130038] (SUSE Linux)


Symptoms:
Mozilla applications compiled with this gcc 4.3 crash on any mouse action performed on their windows, while using only the keyboard works fine. I first saw this on the SeaMonkey development builds I'm compiling myself, and can confirm that an older build compiled when Factory still had gcc 4.2 works fine on the current Factory system (it's what I'm using to post this).
http://download.opensuse.org/repositories/mozilla/SUSE_Factory/i586/MozillaFirefox-2.0.0.9-2.1.i586.rpm also fails the same way, so it's not a failure in current Mozilla development source or SeaMonkey, but related to all those failing builds being compiled with the new gcc 4.3

Mozilla engineers said that "nsEventStateManager::SetContentState has a null mActiveContent, which is... odd" when I showed my stack trace (I have a debug build at hands). We probably end up with that null pointer due to a compilation error.