This problem was apparently the result of Bug 136636. It has seems to have been fixed. I'm running [Mozilla/5.0 (Windows; U; WinNT4.0; en-US; rv:0.9.9+) Gecko/20020411] and am not seeing any symptoms.
[see http://bugzilla.mozilla.org/show_bug.cgi?id=107466 for details] --rt
