The addon Usable Home Button does something similar when navigating, but it jumps all the way up to root level. This addon Navigate Up allows you to step through the directories one level at a time before reaching the root domain. Ah, that's better.
Thanks for upgrading this very useful add-on. Alt-Shift U works fine, but Alt-U doesn't for me at all (hence only 3 stars for now). I'm using FF 56.0 on Mac OS Sierra. Also I like to use alt-up arrow - it would be useful to be able to set our own short cut. Is this possible?