I'll freely admit that i'm guilty of the "develop for chrome, test in firefox" kind of workflow, and I really want to improve on that. I feel like I have built up a kind of "intuition" about what works well and what doesn't over the years in chrome, and i'd like to start building that up for other browsers.
All I know is that I build an app as I think makes sense logically, and 100% of the time it runs faster / less hot in Chrome.
JS-heavy apps burn thru CPU cycles far more on FF than in Chrome.
I want to use FF as my daily driver, I really do. But it's real-world performance just doesn't cut it for me, and I don't think you can blame that on people "solely optimizing for Chrome", as V8 was a better engine at the moment of release, when nobody had had a chance to optimize for it.
Some optimizations are better in SpiderMonkey than in v8, some are worse. Unfortunately, whenever a web developer optimizes solely for v8/Chrome, this hurts Firefox.