Over the years, the Firefox front-end team has made numerous polishes and updates to the Firefox theme.

Firefox 4 was a huge update to the user interface of Firefox compared to Firefox 3.6. A couple years down the line we released Australis in Firefox 29. Next month we’ll be releasing Firefox 40 and with it will come with what I believe to be the most polished UI we’ve shipped to date.

The Australis release was great in so many ways. It may have taken a bit longer than many had expected to get released, but much of that was due to the complexities of the Firefox theme code. During the Australis release we removed some lesser used features which consistently were a pain in the rear when making theme changes (small icons and icons+text mode, I’m looking at both of you

https://msujaws.wordpress.com/2015/07/22/polishing-firefox-for-windows-10/