While we're waiting on the changes to Gecko's CSS handling which will add an at-rule for matching on URL, you can get Chris Neale's new URIid extension for Firefox or Mozilla.
(Chris, can you please make this work for Thunderbird too, or give me some tips on how I can modify it to work there? I really need this for enhancing my Forumzilla experience. I have a three vertical column layout and very few weblogs fit well into the message pane in this configuration. I've got a massively hacked and ugly userContent.css file trying to specifically select on all these horrible table layouts but I subscribe to enough feeds that I'm starting to break one when I go to fix another.)
