"saving space for the content on your screen"
Mozilla is so far out of touch with reality it's not even funny anymore. I'm thinking it's higher ups in the company that make this kind of decisions, you know, the people inside Mozilla who don't know jack squat of coding, because any developer worth its salt knows how important it is to have tools readily available. That's why Photoshop's UI is busy, and Paint's not, or why the new iMac is 5K and not 4K.
I'm a developer, and I use Firefox (what else TBH), and I just updated to version 33 (from version 28, I've been using that since version 29 broke everything), and it took me more than a couple of hours to get everything as I wanted, as I had it in version 28. And this includes coding my own little extension, modifying a couple of them and writing about 100 lines of CSS to get the browser to behave.
Mozilla has forgotten the developers, a long, long time ago. Maybe their developers haven't, and that's why we have been getting new developer tools for a while, but the higher ups inside Mozilla hate us with a passion.
They are willing to sacrifice everything (even their market share, it seems), in the name of more space for the content and preventing the users from breaking Firefox.
And that's not what a developer wants, I'm sorry. That's what people who don't know what they want want. You know, the "average" user.