There is simply no replacement for Firefox (official 64 bit) for my uses. FireFTP is the most convenient and fastest FTP client I have ever used and has one unique feature I've yet to see in another free FTP client. That and a bunch of other extensions that aren't available on other browsers.
As well as Firefox Sync, which works brilliantly between phone, PC, car and tablet.
My Firefox on PC looks like the old skool firefox, not the new theme or anything like that. I use Classic Theme restorer to tailor the look to how it /should/ be, not how Mozilla defaults it to.
I have not found Firefox slow at all on my phone though. It is slow on the Win10 tablet, but that is a Linx 10 dual core tablet with 2GB RAM...