Though not a new hack, I thought I should blog this as it (well, the Thunderbird part) is hidden away in a userscripts.org comment.
When people reply to your LiveJournal posts or comments, you can receive e-mail notifications. It’s lovely that these notifications quote whatever post or comment they are in reply to, but especially for comments on long posts with loads of images, things can get very scrolly and jumpy.
I later realized this could be done in Thunderbird, too. It’s simply a matter of changing your userContent.css to include e.g.
1 2 3 4
Obviously this has some risk of overgenerating, but I have never received any other mail with blockquotes after tables.