Changes

It has a 512 byte 404-redirect threshold similar to Chrome and IE. I've been using http://www.almostinfinite.com/foobar (overrides) and http://www.seattlepi.com/foobar (shows the site 404 page) as test URLs.

A Firefox Extension which overrides 404 pages and supplies information from Google Link Doctor can be found [http://wiki.mozilla.org here]. Note that the 404-override mechanism is currently pretty crude, and it just has a test UI for displaying all the data returned by Link Doctor. I've only tested it on Firefox 3.5, but it should probably work on Firefox 3.0 as well. It's safe to install it on your default profile, but you may not want to leave it installed.