It works well enough for what it is. But there are other addons on AMO that give more features.

Most notably this lacks a preferences dialog where background/foreground colors can be selected which can be found in similar addons like 'Tab Count Icon by lllusion3418' or 'Tab Counter Wide by Jonathon Merz'. I also found the text to be smaller than the other 2 and slightly difficult to read (viewing from about 10ft away on a 50" TV set to 1920x1080); might be better on a pc monitor / different resolution.

FF v89.0.2 on Fedora 33 Cinnamon

It provides an easy way to have an overview of of open tabs and windows.