Customizing something and making it yours is a powerful thing. I think that ultimately that's the reason. One reason is that you make things uniquely fit you. But, I think in more cases than we are aware or want to admit, the emotional reasons are the real reasons. Sometimes its nice to have something you made, not something that was mass produced for everyone.
"Resource hogging" always seems like such a crazy reasoning. It's more aesthetic than practicalities. Is your memory really so scarce?
My computer is noticeably slower when Firefox is running. Quite often, Firefox will grab the audio device and not give it back, silencing all other programs. Sometimes the fan will just spin up and the computer will sound like a hoover until I quit it. Over time it just uses up more and more memory, rarely giving it back. It's not unusual to look at the task list and see Firefox taking up 400+MBytes, even though you're doing very little. (Right now, it's taking up 420MBytes. All I have open is the HN page with this text box I'm typing in.) I also think it does something rude with the GPU, too - when I'm not running Firefox, the auto-hide dock pops up perfectly smoothly. When Firefox is running, though, it stutters as it pops up.
The X11 version is terrible to use over a network too.
(I had a whole different set of complaints about Safari.)
Firefox is the only application that regularly crashes (due to running out of memory) on my system. I could increase memory limits and it'll end up causing so much swapping I wish it had just crashed.
"Resource hogging" always seems like such a crazy reasoning. It's more aesthetic than practicalities. Is your memory really so scarce?