I've been with Phoenix/Firebird/Firefox from the beginning; custom builds, bug reports, tech evangelism, extensions/userscripts; I have made more than one offline XUL application for personal use (JS application programming before it was cool!); the whole ten meters. It had been so good for so long.
In 2008 a few things happened. 1. The extremely sensible and welcome features added in the 2.x release cycle, coupled with the unique browser landscape, ended up derailing the original goals of the project (streamlined browser, minimal yet viable for mainstream use, with robust extension capabilities for anything else anyone could want) back into some ridiculous browser arms race; 2. I switched to OSX and I think the memory problems are even worse there; and 3. Chrome started shaping up to be everything I wanted technically, with its new extension and built-in userscript support (even if it was inferior), its sandboxing, and its sort of remotely sane memory usage, even if it didn't have the warm fuzzy feeling I had from my closeness to the Mozilla project.
I am still so guilty about my switch to Chrome but I spend so much of my life in a browser window that I really had to go the practical route.
And since then it's just been getting worse and worse, with all resources going into either JS performance to keep up in benchmarks or features to be able to add some more bullet points to a release announcement. All anyone wants is better memory management, and then tab sandboxing would be nice after that since Flash/Silverlight can really bring down an embedding process. Give us some core improvements that aren't marketing driven and move all the AWESOMENESS into extensions that can be disabled after install! That's all anyone (on /.) wants.