try Split Pannel Rated 3 out of 5 stars
Split Pannel can split firefox 4 with two panels ..
Only split two,....no more
if you just only want split the firefox 4 to two windows..
can try this add-on before Fox splitter is updated for ff4.
Excellent Addon but do not support FF4 Rated 4 out of 5 stars
Fox Splitter is excellent ,but it's not support Firefox 4..Anyone can introduce another addon to replace Fox Splitter ??(I tried Tile Tab ,but no good><)