I think this post assumes that Firefox is an inferior browser, and that the cause is mismanagement.
But is Firefox an inferior browser? I think it used to be, but over the last couple years, it has made massive improvements in features and performance. I use it in both desktop and mobile, and I prefer it over Chrome.
As a software engineer, I rely heavily on my browser for work. For me, the multi-account containers in Firefox are the must-have feature. No other browser can offer the ease of separating multiple test accounts, and multiple gmail/gsuite accounts for multiple enterprises separate.
Also, I like how Lockwise on mobile is divorced from the browser, making it easy to use it to manage passwords across websites and apps.
Maybe the problem with Mozilla, then, is marketing. Maybe not enough people know that Firefox is much, much better than it used to be. Or maybe the general sentiment is echoed in your post. People don't feel that Mozilla is focused on writing good software, so they don't expect Firefox to be good.
Personally, I think the biggest cause for the loss of market share is simple:
Safari is the default on iOS and Chrome is the default on Android, and population of mobile devices is exploding, and there are no mainstream mobile devices that are carrying Firefox with it.