One of the most common causes of Firefox 'not responding' is that it can't access enough memory because the user's antivirus software is hogging it all as it updates itself.
I've had that problem happen repeatedly (on different computers) when using Microsoft Security Essentials. Svchost.exe is shown, under the Processes tab in Task Manager, as using 1Gb or more of memory, with a constantly changing memory figure for MsMpEng.exe (which is MSE's update feature) indicating that it's MSE that's making the call on svchost.exe.
Switching to Avast has produced (on more than one computer) a similar problem, except that it's then AvastSvc.exe which has the constantly changing memory figure, while svchost.exe again goes sky high for memory usage. Disabling some features of Avast has reduced the amount of time that I have to wait for everything to get back to normal but I've still not found a complete solution. (There are also a few times when it seems to be TrustedInstaller.exe, which is part of the Windows update system, that seems to be making memory-hogging calls on svchost.exe but it's mainly my antivirus software that seems to be the cause of the problem).
So which antivirus program are you running?