According to NetMarketShare stats [0], Firefox is now at under 12% of desktop marketshare... and falling. I've switched over to Chromium (and Safari on OSX) because I just couldn't justify slower speeds and much poorer performance overall anymore. I've also noticed it used a lot more CPU than alternative browsers... which is an issue if you're using a laptop. Once Mozilla kills off XPCOM and XUL-based addons [1], there won't be any real reasons left to use Firefox. Lots of Firefox die-hard users I know have already moved to Palemoon which will support XPCOM/XUL addons for the foreseeable future [2].
It's amazing to me how badly managed Mozilla is these days. They've been on a downward slope for the last two years and they still haven't done much to improve their position. Last week I checked Mozilla's homepage and couldn't even figure out what their guiding purpose is anymore. They seem to care more about social issues than about browsers and technology [3]. Unfortunately, they don't have much future.
[0] https://www.netmarketshare.com/browser-market-share.aspx?qpr...
[1] https://blog.mozilla.org/addons/2015/08/21/the-future-of-dev...