According to the video you can select the icons and menus you want to display in the toolbar... that include all the addons/plugins/extensions too. What customization has been dropped?
The video is deliberately misleading and the claim of "a powerful customization mode where you can add or move any feature" is a complete lie.
A few examples:
Tabs On Top -- No thanks, I want my tabs below the URL bar, where God intended them to be. First, they removed the menu item a while back. No problem, I can just go into about:config and change it. But just removing the menu item and burying the setting in about:config isn't enough. With Australis, even if you go into about:config and change the setting, it does nothing. Firefox says: Fuck You. You're getting Tabs On Top whether you like it or not.
There are only 2 toolbars - Menu bar and Bookmarks toolbar. No more Add-On Bar, which means the Status Bar Extension doesn't work because there's no place to put it. Firefox says: You want a Status Bar? Fuck You. Sure there's probably some hidden fiddling you can do to work around the problem, but that just proves my point -- A user should not have to spend copious amounts of effort just to restore the standard and expected UI.
In Australis the Back and Forward buttons are now glued to the URL Bar, so you can't arrange your buttons any way you like. The Reload Button has been replaced with a swirly arrow that is glued to the far right side of the URL Bar and can't be moved, and the Stop Button is gone completely. Want text labels with your buttons? Firefox says: Fuck you.
There's a lot more, but I give up. If I wanted a browser with a lousy UI that can't be changed, I would use Internet Explorer or Chrome.
Just as I feared -- Firefox has joined the growing list of applications that can never be updated because the new version sucks.