Quantum以降のFirefoxには対応していないようで、紹介されている説明の殆どの機能は削られています。

Does not work and cannot resize fonts as admitted by authors.

doesn't work with quantum.

I am now using Waterfox so that I can use my Firefox add ons. My eyesight is poor now and theme font & size changer is essential for me. However since yesterday I have tried all the versions to download and then turned off automatic updates. The icon is showing in the toolbar, but the problem I have is that when I open the theme font context menu it is not showing any fonts and every time I click on anything it comes up with a box showing Javascript Application TypeError:.

I don't if anyone has any idea what the problem is as without this add on I am going to really struggle to use the internet.

This is an awesome program that makes the unreadable font in fire fox readable. I would rate this app with 5 stars, since the author decided to be shady and add an expiration date i can only rate it at a TWO. Thanks to everyone who made a work around for this amazing app shame the author is shady though. I wish they would allowed us to continue enjoying this app.

not working

reverted to ver 46.5 on thunderbird all now working hope it stays that way

The idea is not bad.
But the terrible behavior, when the add-on simply turns off and stops working without indicating the reasons - is very annoying.
It is understood that it needs to be updated in this case.
What for? It still working fine! I dont need any updates!
It will be easier just to install STYLISH and add a style:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@namespace html url("http://www.w3.org/1999/xhtml");
@namespace svg url("http://www.w3.org/2000/svg");
* {
font-size: 13px!important;
font-family:Microsoft Sans Serif !important;
}

and set
layout.css.devPixelsPerPx = 1

Version 56 of this add-on died today. There are, however, several solutions available. You might want to check out the thread "Theme Font & Size Changer extension for FF57+" on mozillaZine.