I gave it a try, and after 15 years of using Phoenix/Firebird/Firefox I finally migrated to Vivaldi instead.
It also supports WebExtensions, but adjusting the UI is an actual supported use-case (with dedicated support and official fora around it) unlike in Firefox.
It also helped to cut down on the number of my extensions, as mouse gestures and vertical tabs are supported out of the box. One thing less to worry about.
Here is how I configured it on my machine: http://i.imgur.com/jPPVjmP.png
I couldn’t be happier.
It was fun while it lasted.
And so it begins :)
I will miss “It’s All Text!”
I hope they’ll find an analogue, but I doubt it.
I could even live with out an adblocker, but for me it is the “I don’t care about cookies” extension.
You don’t have to live without an adblocker… or other stuff really.
uBlock, uMatrix, Privacy Badger, Cookie AutoDelete, HTTPS Everywhere, Smart HTTPS are WebExtensions already. There’s even Tree Tabs (though it doesn’t hide the normal tab bar for now, the API for that aren’t there yet).
But yeah, I couldn’t find “I don’t care about cookies” :D
Is it possible to add U2F support via WebExtensions? Chromium supports it natively, but FF requires (“legacy”) addons.
I would strongly suspect no. However some pieces of U2F have already landed in Firefox Nightly; see https://bugzilla.mozilla.org/show_bug.cgi?id=1065729 and its dependents. Also https://wiki.mozilla.org/Security/CryptoEngineering#Web_Authentication.