I'm writing this mainly because there are still many people who didn't hear about this: have you tried Refresh Firefox option?
If your complaints toward Firefox are largely about performance this can have major impact, especially if you have:
- Used same updated-over instance for a long time.
- Use a lot of add-ons.
If it's about content, law-thingies or some of the decisions on their part… I have no idea, I'm mainly interested by browser internals.
Just take a deep breath and count to five if your first reaction is going to be a rant on Firefox fans :P. I'm fond of *foxes, but it a rather objective measurement that this one option up there can bring you a major improvement. It speed-up my startup time in my Firefox by a factor of 12 and if repeated about twice a year (not much of a hassle if your add-ons have 'save/restore settings to/from file') you can rather reliably eliminate a lot of the performance drops.