Firefox has always* respected the underline position/thickness in the font. This is why in an example like

    data:text/html,<u style="font:99px times">Times</u> <u style="font:99px Arial">Arial

the Times and Arial elements get different underlines.

(Safari and Chrome don't do this, they ignore the font's properties and use an arbitrary underline of their own -- which differs between the two, Chrome's being somewhat thicker.)

* "Always" may not be strictly true, but it's certainly been a long time.