Last version working relatively well was 5.1.1.2. Version 5.2.0.19 broke many sites including twitter and some forums I use. 5.2.1.2 still has this broken although not as bad as 5.2.0.19 (slows down rather then makes entry impossible). Up until recently you could backlevel to 5.1.1.2. Not sure if its no longer available because of some standard Firefox policy or because of Norton's pulling it.