TrayHotKey is my TOP choice for minimizing Firefox to tray.
Thanks to a hot key (CTRL + 1 in my case) I can minize to tray Firefox and revive it ...
A much better solution for me than "minimize to tray revive" or whatever "minimize to tray" version or solution I tried so far ans stil working with new Firefox version 50 and above (for those who don't know, google "e10s firefox addons"
Working fine with Firefox 50.0.2 (10 dec. 2016). And people looking for a solution to the changes introduced by Firefox 50 should really try this addon.