1881851 - Toolbarbuttons should have a visible border in prefers-contrast/forced-colors
bugzilla.mozilla.org
RESOLVED (mstriemer) in Firefox - Theme. Last updated 2024-07-22.

I am quite happy they added the borders, but I know some people use high-contrast themes as a base for their own theming and don’t want this, in this case, add this to your userChrome.css:

:root {
  --toolbarbutton-outline: none !important;
}