TIP: Setting global default zoom option Firefox 57.0
Not a question, but a tip. I wanted to set the default zoom for Firefox to 120 percent but it would reset to 100 every time I opened it after closing.
Searching through the "about:config" I found:
font.size.systemFontScale 100
which I changed to
font.size.systemFontScale 120
and all is well. Now Firefox always uses fonts 20 percent greater than default, no matter whether I am in a new session or a new tab. Hopefully someone else will find this a useful workaround to the lack of global zoom save option.