I regularly use a ton of tabs. When this extension stopped being supported I edited the CSS file and created new images to support tabs up to 200. Unfortunately, when Australis hit (FF v28.0) the extension stopped working. I can't believe nobody ever came along and updated this for another version of Firefox. It seems impossible that we'll ever have anything like this for FF v28.0+.
There is another number tab extension out there, but it doesn't look as good aesthetically and it isn't over the close button (something I really loved). For anybody looking though, that extension is called "Tab Numbers".
If any extension writers out there would like to do the code for this extension to be Australis compatible, I would be happy to provide all of the graphics. I could easily make graphics all the way up to 999, though I personally have trouble keeping FF from crashing past 300 =P