Rated 5 out of 5 stars
I love this extension and went ahead and just tweaked it one tiny bit to make it FF3.5 compatible. You can get it from here until the official version appears:
Literally all I changed was one single character, the maximum version number compatible. Nothing at all seems to break in FF3.5.
(As an extra tip, there's a FF extension to force incompatible extensions to install: NIghtly Tester tools: http://www.oxymoronical.com/web/firefox/nightly )
See my original comment at the author's website at: http://www.nihongonotes.com/2008/07/04/firefox-fancy-numbered-tabs-extension/?dsq=14922156#comment-14922156