Firefox 130 added borders around buttons in high-contrast themes (finally). Should you not want them, here's how to disable them...
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1881851 - Toolbarbuttons should have a visible border in prefers-contrast/forced-colors
https://bugzilla.mozilla.org/show_bug.cgi?id=1881851
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;
}