I'd just mentioned problems with highlighted links, and someone posted a thread about the same problem. Maybe it's not a single theme causing the problem after all.
The thread:
http://www.freerepublic.com/focus/f-chat/1306879/posts
....and the fix:
https://addons.update.mozilla.org/extensions/moreinfo.php?application=firefox&id=38&vid=1022
Thanks, JoJo, and Merry Christmas.