Life with extjs has been pretty good so far. Although I hit a snag where I couldn't override the icons for tree nodes (see the crappy default file icon above). Turns out you have to do more than just set iconCls, you also have to add !important to your style, so that the style doesn't get ignored and default to .x-tree-node-leaf. I figured this out after taking the screenshot above, so you'll have to take my word on this. Now I have to figure out how to map all the file types in the mozilla tree to famfamfam or something similar (any ideas? I really need .js!).
I also need to figure out how to properly do the search, since there are different types of searches within MXR (free text, filename, etc.), and the other sections of the kit will need to be searchable. I'd value input from those with good ideas.
For the record, the UI of the kit hasn't been my focus so far. I don't know what it will look like exactly, but this is my idea for the general layout. Just imagine it inside Prism and with hidechrome=true (mfinkle, I'm talking to you). My plan is to do the entire UI in content instead of chrome, since extjs is so slick. I'll host this somewhere as soon as it's a bit more feature complete.