In any case, it's probably not a healthy perspective for Mozilla or its community to just write this off as Firefox being the victim of a big ad campaign for another browser. Mozilla had real problems and missed opportunities, and that's where their focus rightfully should have been.
I also use the developer tools, and they're also adequate for vanilla-js development.
So based on 2 decades of experience with Mozilla, I'd go with Chrome surpassing it in market share as more to do with promotion than Mozilla not being good enough.
While your comment prompts me to try again and figure out why this is the case exactly, Google's marketing is not the reason for me (and, unfortunately, others too).
If I had to guess, I'd say it usually boils down to a feeling of clunkiness and weird hogging of memory/processing, whether true or just perceived. I've read some threads that indicate this is a problem for Mac users in particular, but I don't know if this is actually true.
Other developers on teams I've been on certainly pushed it hard (mostly for the dev tools, which I admit I went to Chrome for at times), so there was 'grassroots' support there if I've used that word correctly, whether that came from being hoodwinked by The Great Advertiser I don't know.
I can pop open Task Manager at random times and see Firefox in the 2GB range (this on an 8Gb machine running Slack, Chrome and PHPStorm at the same time, so it's a little cramped for RAM at the best) and I'm running 20-30 tabs. Even chopping down to a handful still keeps you well over 2Gb.
It won't seize completely the way it used to prior to multiprocess, but you get this weird situation where you get a spinner just switching tabs.
For many years, I defaulted to Firefox because of an extension I loved (FireFTP) which could never make the leap to WebExtensions. I can recall developing on Firefox then patching things to make IE6 happy. But on my home machine I switched to Vivaldi entirely.