Great extension, I hope it not drop. If someone will help, there is a small means of partial replacement to this addon.
1. Place/Create/Add next file "userChrome.css" into "chrome" subfolder in your profile, e.g. C:\Users\usename\Appdata\Roaming\Mozilla\Firefox\Profiles\cdar87xx.default\chrome;
2. Fill userChrome.css next content, for example:
* {font-size: 10pt !important; font-family: "Roboto" !important;}
3. It means that you have this font already installed. You can set your own font-size and font-family values, ofcourse;
4. Restart FF.
Now it looks closely than I has with TFSC.