How do I increase the font size of menus, tabs, and other user interface elements in Firefox 52?

The suggestion to let layout.css.devPixelsPerPx control UI font sizes on Linux was rejected.

That same bugzilla page recommends to control the UI font sizes with the GDK_DPI_SCALE and GDK_SCALE environment variables.

For example, if you're starting Firefox via a launcher shortcut, you can change its command from firefox %u to env GDK_DPI_SCALE=1.25 firefox %u

While trying to solve this problem I learned about userChrome.css. It is a configuration file allowing users to change properties of the Firefox UI, using the cascading style sheets language. It works on Firefox 52.

The file does not exist by default; I created it and added the following text:

$ cat ~/.mozilla/firefox/*.Default\ User/chrome/userChrome.css        
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

* {
font-size: 16pt !important
}

Don't forget the semicolon after the @namespace line.

Amazingly, there is a whole website, userchrome.org, dedicated to things you can put in this file. However, the above simple snippet was all I needed. I didn't get it from userchrome.org but from a post on mozillazine.org. On my system the font-size trick is also to be found in a file userChrome-example.css in the same chrome/ directory.

Now I can read the menubar text and tab names and so on, with my screen at full resolution.