One data point: I’ve been building Web GIS applications for the past 15ish years and Firefox has one and off been a real pain in the neck, suffering from canvas API quirks and performance issues. I think the current state at work is that Firefox is palpably slower than Chrome and I need to find time to go figure out why.

Another point: I’m not sure if Safari’s dev tools are worse but I feel friction any time I have to use them. It’s probably partly just that I don’t want to learn the quirks of both sets. Firefox’s is much closer to Chrome’s. I bet they’re both inspired by Firebug.