It looks less awful than the current theme in Linux, thanks to the menu button (which usually occupies the position of the leftmost tab in Linux and isn't even orange) being moved to the same location as where Chrome has it. But there still seems to be a lot of wasted space in the titlebar of the Linux theme.
I remember reading that this is because certain opendesktop UI guidelines (can't remember which one) prohibit apps from encroaching on the window chrome. But Chrome doesn't care and puts tabs at the top of the chrome anyway, and this is quite useful because you can just push the mouse to the top edge of the screen to click on a tab. With a thick titlebar occupying the edge, on the other hand, you need to aim more carefully.
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.
It looks less awful than the current theme in Linux, thanks to the menu button (which usually occupies the position of the leftmost tab in Linux and isn't even orange) being moved to the same location as where Chrome has it. But there still seems to be a lot of wasted space in the titlebar of the Linux theme.
I remember reading that this is because certain opendesktop UI guidelines (can't remember which one) prohibit apps from encroaching on the window chrome. But Chrome doesn't care and puts tabs at the top of the chrome anyway, and this is quite useful because you can just push the mouse to the top edge of the screen to click on a tab. With a thick titlebar occupying the edge, on the other hand, you need to aim more carefully.