Yeah I've tried that many times (I have some experience with vim, gcc, windowmaker, but mozilla was larger than them all, last time I checked) and with some success. It's totally possible. My point is merely that it's labor-intensive. There's a cost to swapping in all that state, and to swapping it out and back in again everytime you find a bug with your hacks.

(Tangent) Lately I find I'm much more likely to hack on codebases with lots of tests. Having a single command tell me when I've broken things makes me less afraid of having to deal with subtle breakage, where my changes break some feature I just happen to not use for the next n weeks/months.