Apparently requested since 2004, according to gHacks[1], but the oldest entry in Bugzilla I could find was from 2014[2]. However, that entry does reference a post from mozillaZine from 2004[3]. Though it's not really a request, but an addon that made it possible to change keyboard shortcuts back then.

1: https://www.ghacks.net/2025/11/17/firefox-is-adding-support-...

2: https://bugzilla.mozilla.org/show_bug.cgi?id=972662

3: https://forums.mozillazine.org/viewtopic.php?f=48&t=72994