To be honest, I really don't care about the UI on Linux so much as the terrible lag. For whatever reason, Mozilla just can't get its shit together when it comes to fixing lag on Linux. For this reason alone, I continue to use Chrome, even though I'd love to move back to Firefox and get all the customizability that it's known for.
I'm not sure what you mean by lag, chrome has preloading, possibly that's what you're seeing (negative lag?). I use Ubuntu 12.04 and don't see any "lag". FF is snappy. I use chrome most, but mainly because I prefer it's developer tools and memory footprint, but neither of them are very economical with memory.
Can you try "resetting Firefox"? You can go to about:support in your browser and click on the Reset Firefox button. This will fix most, if not all, lag issues you are experiencing.
