Another user seeing this: https://www.reddit.com/r/firefox/comments/fxvco9/changed_to_dark_mode_on_my_mac_and_now_certain/
Bug 1588847 Comment 11 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Another user seeing this: https://www.reddit.com/r/firefox/comments/fxvco9/changed_to_dark_mode_on_my_mac_and_now_certain/ about:support in comment 12.