It's there, but you probably can only see it when you switch to full screen mode. Do you see "Preview" (bottom right) in this screen shot? That's where you have to click after you make your changes.
I can just barely do it when I hide my lower (add-on) Firefox toolbar (I use a laptop). Once I click "Preview" I get a smaller box where it's easier to click "Save". I've added Gato and changed "Ten" to "Eleven".
It's easier to do using the Google Chrome browser, because there's no add-on toolbar. You could try using another browser. Edit signature--->Preview--->Save.