Really, it was far too easy and a little disappointing that I needed to bother, as all I needed was a simple way to hide the browser chrome when I wanted a little more screen space for the content or I wanted a distraction-free environment for reading an article.
I wrote Focus Mode for Firefox to do just that :)
Now, someone tell me why that’s not already a standard feature in Firefox. Or even better, tell me that it is and that I just failed to notice it. And while you’re at it, tell me why I couldn’t find an existing extension that does it!