The biggest thing to learn isnt in the patches. It's in the process -- don't be afraid to break things, and dont let "I don't know everything about this" stop you from learning one small corner of it.
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.
