I always found Firefox to be unreasonably slow and resource demanding for basic web surfing: right now, it is using ten times the memory that Epiphany uses (I am looking at the system monitor with both browser open, same amount of tabs, same websites, Firefox sync disabled and the only Firefox extension I have is uBlock).
Firefox, while nowhere near as popular as Chrome and its derivatives (Chromium, Opera, etc.), is still a fairly well-known cross-platform browser. Most websites include Firefox in their testing, which means that you rarely end up with incorrectly rendered content; whereas with Epiphany, you'll sometimes run into issues (in particular, I find that HTML5 videos will occasionally crash the tab without apparent reason). Hence the generally positive attitude towards Firefox, and the occasional complaints about Epiphany being pre-installed.
Personally, I prefer Epiphany mostly because of the design—it looks and feels at home on elementary OS, and it is sufficiently advanced for my daily usage (although I do miss Reddit Enhancement Suite and one or two other plug-ins I had installed back when I was still using Opera… I really wish the devs would get around to re-implementing plug-in support in Epiphany, at least for userscripts).
That said, Mozilla have been hard at work on a lot of improvements to Firefox, combining various techniques from different browser engines into one new, state-of-the-art engine. They're going to start switching out different components in the coming versions (as far as I'm aware, the current nightly build already includes the new CSS module, although it might be toggled off by default). If all goes well, Firefox should rise from its current, rather unfortunate position back to the top of the performance race over the next year or two… it'll probably still use a little more RAM in idle than Epiphany, but that's pretty much unavoidable, given that it has more features built-in.