Adblock is back, and packs more punch than before!
A new official build (0.5 d3 nightly 40) of adblock is available at adblock.mozdev.org. This version is compatible with Firefox 1.5, and sports a new trick: it blocks ajax. This means you can use adblock to disable Gmail ads. Here's the filter you can use:
/mail.google.com\/mail\/.*view=ad/