I doubt it makes a lot of sense but it would be nice to see how Firefox compares on different platforms:

Firefox 15 x86 on Windows 7 x64
Firefox 15 x86 on Ubuntu x86
Firefox 15 x86 on Ubuntu x64
Firefox 15 x64 on Ubuntu x64

Firefox nightly x86 on Windows 7 x64
Firefox nightly x64 on Windows 7 x64
Firefox nightly x86 on Ubuntu x64
Firefox nightly x64 on Ubuntu x64