To be honest there was the same kind of talk when FF2 was in beta, then most people just moved to it anyway. People say the same thing every time there is a new version of a tool like this "oh no bloat!" "oh no, design over substance".
What is there for FF since all the functionality is in the plugins? FF has become a web-rendering widget engine, but that makes the core product weak, and they are working on strengthening it. Makes sense to me. Don't rely on the add-on makers for basics anymore.
It helped early on to be tiny and lean and leave it all to the add on writers. That serves a geeky crowd well. Once you go for more market share (they didnt dream of that back then) this approach starts to backfire. Normal people installing FF just wonder what the fuss is all about, it is a "bah" product out of the box with no plugins.
Also a lot of functionality which was earlier done in things like add-ons, we now know how to do on the web, in a cross-browser way. So again, people wont install an add-on they will use a web app, and FF is again under pressure, since the advantage some of these add-ons gave is going away. And it has had as many security holes as anything else recently, so that advantage is gone.
I left FF after 1.5 - FF2 just rubbed me the wrong way, it was more instable and of course the existing plugins didnt work so I had to use plain FF again, and that is when you notice that FF is actually pretty lame without the plugins. I didnt want to use it plain, neither did I want to spend hours hunting plugins.
Also by then the FF community was starting to become really annoying and arrogant, so I bailed out. I dont like fanatism about anything, and it certainly is totally out of place on mundane things like software. Whatever it is you feel "evangelic" about, it's probably not that great or that important that you really ought to go around telling people to repent from their own software and use yours. ugh. I dont like FF only sites (eg: "Sorry, but we do not support the Opera web browser. However, Stumbleupon works well with Mozilla-based browsers, free open-source ones based upon Netscape Navigator. Our Toolbar doesn't work with Opera because it relies on browser features which Opera does not have. Mozilla and Firefox are ad-free, fast & stable, with banner/popup-ad blocking and tabbed browsing features. Install one now before joining the Stumbleupon community" Yeah, right.)
Most enthusiastic FF users do have a really good browser that works exactly like they want it to, but they spent 200 house over the last 3 years getting it that way. And they forget that.
I used slimbrowser, used kmeleon, and now opera. I am a lazy user and I spend a maximum of 1 hour over 3-4 sessions tweaking a piece of software. That's it. And I dont particularly like the plugin lottery.