When I used Firefox with KeePass, I relied on the KeeFox extension. The handling of multiple accounts for one page is probably a matter of taste. It wasn't quite what I hoped for (even though KeeFox - or Kee - is all in all superb).

Having worked with Vivaldi for a while, I've noticed that ChromeIPass's approach is more to my liking. So, when I tried out Firefox Quantum Beta, this extension made me go back to Firefox full-time. It's simple, lightweight, and works as expected.