That'll teach you to mess about in preferences and not take notes about what you did so you can get back!
(no, sadly, it probably won't :lol: )
Here's what you did: you went to Firefox's preferences, and under Language & Appearance you clicked on "Colors" and changed the "Override the colors specified by the page with your selections above" to "Always." You should change it back to either "Never" or "Only with high-contrast themes."
Cheers
Chas