I would agree its currently the worst for the simple reason it cannot even scroll pages smoothly and play videos smoothly, these are very basic performance expectations from a web browser that should be got right before everything else.
For those who dont want to read the long page or dont understand it the bug is to do with 'garbage collection', basically in V3 firefox decided to work on reducing the memory consumption in firefox as v2 had a rep for leaking memory. The changes included making the caching go to disk instead of ram and flushing the memory at set intervals (garbage collection). When loads of tabs are open and especially ones with multimedia content then this flushing causes firefox to stutter.