I have a similar problem, albeit a bit different. Everything worked fine with my Firefox on all my computers. All of them were sync'ed with firefox sync (including my mobile). Then, on my Windows 7 Home Premium 64-bit desktop (Q9450 CPU, 4Gb of RAM, Asus P5KC motherboard Intel P35-based), I replaced the motherboard and the CPU. Obviously I reinstalled Windows from scratch and Firefox as well. From plugins, I installed only Adblock plus with the Easylist and malwaredomains lists. The same plugin (along with others) were installed previously on my desktop. I initiated a new sync and my bookmarks and everything obviously appeared ok. On this new hardware platform (Q9650 CPU, 8Gb RAM, Asus Rampage Extreme, Intel X48-based) I now experience strange freezes, from 10" to 30". These happen while I am trying to do an action, like fill in a form for example. Firefox does not freeze while it is idle. When this freeze takes place, it affects only firefox. Nothing else gets affected. I think there is one other application that exhibits these freezes, a Fujifilm photo retouching application. Can't be sure though. It drives my crazy because both the motherboard and the cpu are better than the one I used to have. The system is not overclocked. All my other firefox installations work perfectly. Being a sysadmin, it all might indicate a hardware issue and not a software one. However, since firefox is the one to exhibit this behaviour, there is some correlation here. All help will be appreciated.