I agree that it's hard to discover, as I haven't been aware of it until today, despite the thickness of firefox's UI being the most major grip I have with it as

I agree that it's hard to discover, as I haven't been aware of it until today, despite the thickness of firefox's UI being the most major grip I have with it as an otherwise ardent user. I've added it to my configs and will find a CSS solution if the option goes away.