The Split Browser Firefox extension (by Shimoda Hiroshi) was actually mentioned by dk70 a while ago in a thread on favorite ff extensions.
It's a very cool extension that lets you tile multiple tabs within the firefox window.
I use tabs heavily, and the Tab Mix Plus extension is my daily companion. I love tabs. But occasionally you want to put two things side by side or have a few pages open and visible simultaneously. That's where Split Browser comes in. Packed with options it lets you split your window into multiple panels, resize and merge them at will. Very useful and works very well.
Note: there are a few other extensions that do similar things -- I haven't tried them all and for all i know there may be better ones.. Anyone want to chime in.