"Arrgh! Firefox freezes every 18 seconds -- 100% CPU utilization for no reason I can ascertain. Leads sometimes to odd stuff like the above if one is not careful...)"
I've seen the same thing, though not quite every 18 seconds.
But it's getting annoying enough that I may drop Firefox for Opera. At least on Windows. It isn't happening on Linux, but the version of Firefox with Suse 10.0 has other problems.