I don't know if this is a rendering error or evangelism. Image follows.
Generally not a good idea to file Layout bugs without testcases as NEW in Firefox:General... Have a regression range? Testcase?
Sorry, I was trying to finish up my work week before Xmas. Next time the bug happens I'll try to reduce a testcase.
Slashdot has switched to a CSS layout, and different advert layout. Problem no longer occurs.