The current build of Firefox for Android is being sunsetted soon, last noon security update is going to be in July. The version they are moving to, Firefox fenix is very fast. At the moment I don't think it has sync enabled (so no sign in) but the future looks good there.
Nice maybe Ill go back for Android. I couldnt figure out why it was so slow, its seemed almost like a DNS issue because the first page load, even if it was local, took 10+ seconds. Im a huge FF fan and supporter, I'd like to go back.