Great add-on but needs a little TLC Rated 5 out of 5 stars
I love this add-on and have used it for months, but it does have issues. It seems to work perfectly with some themes, such as PitchDark, but with others, like Chromifox, I get what Mia Junk reports - four icons, rather than one. They're the four versions of the icon, but instead of changing with their function, they're all shown lined up together (as in #XX#).
Some themes, such as Aquatint Black Gloss and Aquatint Redone treat it differently still, in that there seem to be more than one icon as above, but only the center of it is shown, so it looks like two different halves. Unlike Mia Junk, I do not have tons of add-ons installed, and the tab-related ones I have are Ctrl+Tab, Duplicate Tab, Submit to Tab (disabled), Tab Counter, Tab Scope, and Undo Closed Tabs.
Disabling Fancy Numbered Tabs, restarting FF, and enabling the add-on again, and other work-arounds don't seem to help. I currently have FF 3.0.4 but I've had this issue with previous builds. Regardless, it does seem to work properly in any state, so this is primarily a cosmetic issue, albeit a pretty annoying one. I hope this is fixed and updated soon. In the meantime, I may try the two newer tab numbering add-ons.