Firefox was just getting too heavy for me. I'm on Linux, (Mint 18 XFCE) and it was taking 30-45 seconds to become responsive after launching. It would just sit there. Even if I launched it from the command line with a url, it refused to do anything for that time period was up. CPU and memory were not taxed or even being used by FF. I have an older processor, but plenty to handle a damn web browser. (Intel Core2 Quad Core, 8GB of RAM) Unless I open a ton of tabs and GIMP, I barely ever get past 4GB used. I have used FF almost exclusively since at least 1999. I went to Chromium for about a year a while ago, but came back to FF.
After a few months of putting up with its freezing issue, and hoping updates would fix it, I just had to quit using it. If I left it open, I would notice that the CPU would spike for several seconds on occasion, and hang out around 20% for a while. While no page was loaded. I could only put up with it for so long.
I have a few other browsers installed... Don't really like Chrome or Chromium. I like certain specific things about FF that other browsers don't have, at least not in the way I like them. Then I found Pale Moon , and it seems to fit the bill. There are still a few things I would like to be able to customize better, but so far it's the winner in my book. That may change, I don't know. But FF seems to just keep pushing me away.