Firefox-now: TreeStyleTabs + ok browser
Chromium-now: fast browser
Firefox-future: ok browser
Chromium-future: fast browser
As long as TreeStyleTabs > (fast browser - ok browser), then Firefox is a better choice. Without TreeStyleTabs, Chromium (or Chrome, or whatever fork is better at protecting privacy) is the better choice.
I believe one of the key reasons for dropping XUL extensions is because they don't work very well with e10s (multiprocess Firefox). I recently had occasion to use a fresh Firefox without any e10s-incompatible extensions installed, and it really felt much faster, so I think Firefox-future may also qualify as "fast browser".
But Mozilla claims that it's exactly the old add-on system that keeps Firefox from being more secure. What if they were just as secure as Chrome after giving up on the old add-on system? Wouldn't it make more sense for them to compete without the disadvantage of being the less secure browser?
I get the feeling that a lot of users really don't believe XUL, etc. is inherently less secure. I cannot say either way myself, but without the capability of the old plugin system Firefox has no real advantage over Chrome. It probably doesn't help that they are going with something compatible with Chrome which leads to a "why not just use the original" thought process.
I get the feeling that they could have got the new system done and as capable as the old before deleting the old. Transition periods are brutal as Python and Perl have found out.
That's not really true. Chrome is more secure because it's designed better, they have very good and mature sandboxing for the parts of the browser that work with data from the internet, and because they work very hard at improving their codebase. They have their own LLVM fork!
WebExtensions can still read all your data on all websites and inject whatever they want into all websites. Not a lot of security there. That is OK though, I am fine trusting Google and Gorhill instead of just Google.