FireFox is FAR more flexible than GIMP, but that comes at a cost of speed.
You can actually speed FireFox up with a custom build (an option to configure for FF actually) or by uncompressing all the chrome for firefox before hand. In theory it shouldn't matter as FF caches it, but it does make a difference.
The main thing is that they aren't shooting for the same targets. Chrome is trying to be as fast as possible so Google's webapps run fast and feel more like native apps. FireFox on the other hand is trying to be far more flexible.