Would have to be with a very noticable warning. Other than that, it could use TOFU like Gemini.
@Hyolobrika
It almost works like that already.
When you open a page on a server with self-signed cert, it gives you a warning, if you accept it, it adds an exception for that cert — you can see the list in preferences under Privacy & Security → Certificates → View certificates → Servers
@Hyolobrika
No, it's Mozilla's generic white on #1C1B22 (no idea what the name of this colour is) — but you can make it you way with userContent.css — and I never tried making these myself, but I'm pretty sure you can make an extension that does this — or, look it up, maybe someone already made one 🤷