I got the same 100% CPU usage by process avastsvc.exe, so Firefox isn't a problem, Avast is. It's obviouse, that Avast has some bug or incompatibility, and seems that support team don't want (or can't) propose any solution. There are some posts in different threads, and seems that same happens also with commercial versions (for free version you can't expect any real support).
I have W7 Ultimate 64-bit, i7 CPU and 6 GB RAM, so I really don't like when it beheave as an i486 PC.
I have IE9, Firefox and Chrome installed (all latest versions), no additional firewall or antivirus installed. I have Adobe Acrobat Pro 8 installed (there were some reports about conflic with pdf printer, but I don't see such problem).
Problem started after some recent update of AVAST, I haven't seen it with v.5, but I see that some other users had it with v.5. Anyway I tried to update to latest version and definition, but it's already up to date, so no success.
I tried to disable some parts of Avast, as was recommended in support forum, but once the process got about 100% of all 8 cores, it doesn't go down whatever I do.
So I had to uninstall AVAST and swap to AVG. It's a pitty, as otherwise AVAST was nice and convinient. It is interesting that on the first scan AVG had found 6 viruses, which AVAST has not found, in some old compressed installation files. May be they are false positives for marketing reasons, may be not.
If there is any solution for the problem, I would be happy to switch back to AVAST.