I considered switching to Lunascape, which is a browser that will let you use webkit rendering and firefox plugins, but it's Windows only :(
I'm in exactly the same situation - I literally cannot stand to use a browser without that extension anymore, especially on a widescreen computer. I've tried to move over to Chrome, but the lack of that extension is the single blocking issue for me.
Step one when I get a new computer is to install Firefox; step two is Tree Style Tabs. Anyone that hasn't tried it is really missing out...