Have a look at the trends[0] and tell me it does not show a descent to darkness.
1. Firefox was at ~30% market share at its apex (it's now half that)
2. It got surpassed by a newcomer with no previous experience in browser making in less than 3 years
3. And the newcomer was so much better at technical stuff and marketing that it not only kept superseding Firefox but it kept growing uninterrupted ever since
3. Meanwhile, instead of stabilizing and reversing the trend, Firefox's usage share just kept tumbling down to the level of Internet Explorer's (Internet Explorer!)
4. It. Never. Recovered. Seriously, it had around 12% market share a month ago[1], below IE's share. That's ridiculous.
Of all those points, I think maybe #4 shows Firefox's irrelevancy best: the most generous estimates now place it around 14% global market share, barely above while Chrome is striving at around 60%. If IE is becoming more and more irrelevant (it is, right?), then surely Firefox is becoming just as irrelevant as well.
Even though the data isn't ideal and there are probably some +/- percent point errors between Netmarketshare, W3counter, Alexa, Google, etc., the trend is identical among all these reporting platforms.
0: https://en.wikipedia.org/wiki/Usage_share_of_web_browsers#/m...
1: https://netmarketshare.com/browser-market-share.aspx?options...