ChrisSD: Um.... The old UI was nowhere near fully customisable. Addons helped tho.
Trilarion: It's the same for me. But then I think that I'm particularly insensitive to bad graphical design.
The problem is that with every version after the journey to Australis stable started, they're taking out functionality; the number of add-ons you now need for things that worked natively before increases with every new version, and with all the code that is being taken out it's sometimes not even possible - for example, I hate tabs on top, but the about:config hack to disable it doesn't work anymore. I always regarded add-ons, about:config (i.e. prefs.js and user.js) and the userChrome and userContent files as means to enhance functionality and to customise the browser to the usability and looks that suit me the best, but a lot is negatively changing, like the dropping of the chrome folder in your profile a good number of versions ago.
Not to mention that the add-ons update function behaves in a broken way - I prefer to manually check and then decide to download and install them, but it doesn't work like this anymore.
I got tired of spending time and energy trying to restore things that they took out and moved away from native Firefox around v.14 or v.15 (don't remember the exact version), made
Pale Moon my default browser (with the majority of my add-ons being security/ privacy related) and never looked back. Not only that, but I have also moved family members and friends over to Pale Moon because, imho, it's currently the best Gecko based browser for Windows users.
And this comes from a once hardcore fan of Firefox - once, when Firefox used to be the browser of choice for folks that enjoyed to fiddle with its settings and functionality; now it has become more suited for the average user that either doesn't know or doesn't care to bother with such stuff. Sadly, as others have said, Mozilla has chosen to go forth with this transition with Firefox as a poor Chrome rip-off/ wannabe that not only won't ever manage to convert Chrome users, but is also loosing fans to other Gecko based browsers.