If you're using Waterfox, that add on will change back to the "new" version every now & then. I just remove it, go back into "Get Add Ons", find Theme Font Size Changer, then scroll down to "All Versions" & select Version 56, then add back to Waterfox/Firefox. It's a pain in the butt but at least not too hard to get it back. Really wish they would get rid of the latest version & just go back to the Version 56 but I guess that one does not work with Firefox Quantum, so it is what it is. Apparently the developer of that add on does not want to bother fixing it to work with Firefox Quantum.

OK - I stayed at FF 56 so I could use Theme Font & Style Changer; this AM it quit working and the icon is not on my menu bar. Reinstalled TF&SC 56 and no go. Has it been permanently disabled? Mac OS 10.11.6 If so, how do I get my toolbar size to increase and my bookmarks to increase in size so they are readable? Or, is there a way to "reactivate" TF&SC 56 (as apparently it has "expired" due to some coding)? Again - I'm on a Mac OS so none of the Windows 'fixes" will work; spent all morning trying them all.

I suspect it was Firefox that did it. Supposedly these updated versions are "scheduled" to only last just so long, then are disabled. I have opted to go a different route to getting the font larger on the tabs, etc. If you check the response I got from jscher2000, on 1/5/18 @ 8:30:01 AM, there are instructions as to how to do this. It gives you larger print in that area, but you still can't choose a different font or font color. You also end up having to re-adjust the size of the webpage as well.

There seems to be a coordinated effort underway to stop anyone from using theme font and size changer - by both sabotaging the add-on itself with a self-destruct timer, and removing the instructions for how to FIX the problems from any forum connected with mozilla.

I found (again) an explanation and instructions on how to fix this at: https://www.reddit.com/r/waterfox/comments/7oeh8z/theme_font_size_changer_addon/

I suggest anyone who wants this information should google the name of the add-on and "fix expiration date". Frankly, the hijinks being pulled around and through this issue make me VERY leery of using Firefox for anything ever again - can't trust them any more.