Created attachment 124047 [details] Example screenshot The Breeze theme is not being properly applied to some items in Firefox (buttons, checkboxes, radio buttons, sliders, etc). Although the Breeze theme is also set for GTK applications under System Settings - Appearance - Application Style - GNOME/GTK Application Style, the appearance of those items doesn't match the theme. This seems to have been introduced by a recent update to Breeze GTK as it was working correctly last month. I attached an example screenshot from one of my web apps which uses simple checkboxes. You can see a gradient blue box with the "V" symbol in the middle: This is not what the checkbox of the Breeze theme looks like.
This is not an issue with Breeze GTK, but rather an intentional choice by Firefox to always use Adwaita as the content theme. However you can override the theme in about:config. See https://askubuntu.com/a/1037112 for information on how to do this.
(In reply to Carson Black from comment #1) Thank you: That has solved the issue. It happened after a distro snapshot that also changed the Breeze theme, so I thought something broke and KDE was failing to properly inform Firefox of the correct theme being used.