Please Deflate your UI

 The last one to do it are Firefox people (dropping "compact" layout), but let's make it clear: using pixels of our screens (especially vertically) is becoming increasly difficult as touch screen get prevalent. I've been trying to avoid Adobe Acrobat Reader DC for as long as I could for that very reason too.

Please, once for all, make the padding-around-icons/text/UI elements a system-wide tunable feature. Stop having padding pre-built in your icons. It is not something you as a system designer should decide, just like you shouldn't decide whether I'll wear a tie or a bow. 

Look at that non-sense: there is so much ribbons and margins when I have a document to review that none of my screens would allow me to properly see an A4 page large enough so I could read the text effortless... unless I turn full-screen of course, in which case I no longer have access to people's comments.

(I truly wish there was an open-source alternative to adobe review system, but given that they now try to automatically upload reviewed documents on their cloud, I doubt that is about to come)

Hopefully, with Firefox, you can toggle browser.compactmode.show and have 'compact' option showing up again when you try to customize your menu bars.