I have the latest FF, that's when it started. I rolled back to version 11 then opened FF and it went right back to version 12, updated itself. Funny thing is it doesn't happen right away. See reply #4....same thing happens to me regardless of what version of FF I use. Always did that but only on WC. Doesn't happen on any other site.
One more thing to try: a new profile. Your profile is what stays constant across reinstalls, and it's what is different in your install from others.
Start firefox with -profilemanager to creat a new one.
This will have you running without anything that may have been corrupted in your profile or that you may have changed without noticing. It also starts without addons.
Oh, and if that also doesn't help, you may want to try a beta/aurora/nightly build to see if a future version will get better. Even the nightlies are usually stable and usable.