Nice, but apart from "Close" and "Mute", I suggest "Reload", so that the music really stops but the tab is not lost ("Unload" would be nice but I get into trouble).

I also made a few minor improvements. Here is my modified version, feel free to incorporate the modifications if you like.

garetgv.fr/vrac/sleep_timer_fix-1.0.5.1+reload_signed.xpi

Thanks for fixing the original. This version always works without issue.