firefox is my second best browser after chrome. i keep both installed and both are great but i simply have loads of RAM.
The best browser gets even better
And by the switch to "web extensions" a HUGE part of the existing add-ons is no longer working - so I would recommend not being too hasty in upgrading. I have put my FireFox on manual upgrade, until a larger part of my add-ons are upgraded - and some never will be!! ;-(
Some of the crucial like NoScript is not ready yet, (Though it's announced that it will come)
Sure for a FEW you can find alternatives, but for many you can not.
Best browser ever!!! Speed is not everything...