Firefox and UI font size increase issue

If the about:config setting of layout.css.devPixelsPerPx persists, but has no effect after restart, this is most likely a bug in Firefox.

Before signalling the bug, try disabling all extensions in about:addons, to see if this is caused by bad extension. See also How to Uninstall or Disable Extensions in Mozilla Firefox.

Otherwise, while waiting for the Firefox developers to fix the bug, you could try using userChrome.css. To enable userChrome.css and userContent.css, you need to set in about:config the following:
toolkit.legacyUserProfileCustomizations.stylesheets = true.
See also How to Create a userChrome.css File.

Try adding this to the file, adapting it to your needs :

/* global font */
* {
  font-family: DejaVu, sans-serif !important;
  font-size: 13pt !important;
}

More information in the article Firefox/Tweaks.

You could also try the more extensive global UI fix from Github: Firefox-UI-Fix.