A great feature to have in Firefox would be to be able to view 2 or more tabs side by side.
I'm not the first person to want this. This guy's mocked up a screen showing 2 tabs side by side: User:Tuomaz - MozillaWiki
If you just want to see what tabs you've got open together on a page you can use the FoXpose extension. But this just provides an thumbnail image of the tabs with an option to click on a thumbnail to go to that tab. The thumbnails aren't actually usable though, they're just images.