[boomboomsubban]

I have a wild guess. Windows changed the default font to Segoe in Vista, which was released eleven years ago, and you are not a fan. I'm guessing six years is a rough guest or you stuck on XP, or dealt with this problem and forgot you did it years later when you reset settings.

That said, I searched "Firefox Segoe" before writing this and the top few results were from about six years ago when some naming problem caused Firefox to use the wrong version of Segoe. Worth checking on your own.