I suspect this is caused by this new feature in Firefox 91: https://www.mozilla.org/en-US/firefox/91.0/releasenotes/ > Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS (This is bug 1713015) You can disable this in Firefox's preferences - see https://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we#w_overriding-page-colors , "Overriding Page Colors". You want the value "Never". See bug 1725978 comment 4 for more details.
Bug 1725341 Comment 3 Edit History
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I suspect this is caused by this new feature in Firefox 91: https://www.mozilla.org/en-US/firefox/91.0/releasenotes/ **Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS** (This was added in bug 1713015.) You can disable this in Firefox's preferences - see https://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we#w_overriding-page-colors , "Overriding Page Colors". You want the value "Never". See bug 1725978 comment 4 for more details.
I suspect this is caused by this new feature in Firefox 91: https://www.mozilla.org/en-US/firefox/91.0/releasenotes/ **Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS** (This was added in bug 1713015.) You can disable this in Firefox's preferences - see https://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we#w_overriding-page-colors , "Overriding Page Colors". You probably want to choose the value "Never". See bug 1725978 comment 4 for more detailed instructions.