From chatting with PJ's Little Brother, I realized that I never weighed in with an opinion on Firefox here. Not that anyone cares, but... y'know... you'd think that I, of all people, would have something to say about it.
I tried Firefox for a few days and that was about as long as I could stand it. I think it could be a good browser when it grows up, but I'm going back to Mozilla for the time being.
There was no appreciable decrease in the memory usage from the Mozilla browser-only install on both my machines, home and work. This is my biggest beef with Mozilla. I've seen it go up to 100M+. When it was still "Phoenix", Firefox was actually far worse than Mozilla. I'm not sure if it was a whole order of magnitude, but it was enough that even I was disgusted. But now, it's merely just-as-yucky, which doesn't impress me either.
I have a Mozilla custom keyword 'lj' that maps to http://www.livejournal.com/~%s. To go to a journal, I type lj <username> in the address bar. Because I know how the views are named in LJ, I can also go lj <username>/friends?show=P. This jumps to anyone's Friends view. If I'm in the Friend List, my entries will show up there. I do this often to check my formatting and colours in other peoples' layouts. I do weird things with my entries sometimes and I casually care that it doesn't look totally whacky-dooky to you.
Firefox automatically converts to the URL-encoded format for keyword substitution strings, so what I type gets expanded to http://www.livejournal.com/~<username>%2Ffriends%3Fshow%3DP which, of course, 404s on me.
Type about:config in the Address bar. *SWOON*
Anyway, I've played with the network.standard-url.encode-utf8 and network.standard-url.escape-utf8 settings to no avail. It always converts all characters in the %s substituted string that are outside of A-Z, a-z, 0-9 ranges. Mozilla only converts spaces and leaves other ASCII-range characters intact.
There are no real bookmark groups in Firefox. They're implemented as just folders that you right-click on and move down to "Open in Tabs". This is option is at the bottom of an arbitrary length menu, which means I can't really rely on muscle memory.
In Mozilla, I can single left-click on the bookmark group and they'll all open in separate tabs.
I go by muscle memory a lot and the difference in context menu item order for the Tabs is pissing me off. The "Close Other Tabs" in Firefox is where the "Reload All Tabs" option is in Mozilla. I keep closing all tabs except the current one when I want to reload them all. And there are other things that are in different places too and I keep poking the wrong thing.
It doesn't support an explicitly linked shortcut icon in the stylesheet. It's pointing at the shortcut icon for the root of the domain and not at the one referenced in the stylesheet.
And — yes, this is almightily important to me — there's still only partial support for Devanāgarī ligatures in Sanskrit.