I emptied the cache but didn't see any improvement. I also did a full uninstall-reinstall to no effect. At Jason's suggestion I went to the firefox help forums and did find a few posts about the same problem. I toggled a "network pipelining" setting that I had forgotten I even activated, and turning it off seems to have helped. It's still a bit screwy but not nearly as bad as before. Good enough! The problem has nothing whatsoever to do with GH.