There are tricks to make it faster. If you find yourself working on the project again, ask for advice on IRC. 2-3 minutes is longer than it should take in most cases. (As long as you aren't touching widely included headers files, that is.)
It's a balance, but on many projects the overhead of the headers themselves is so large that concatenating a few .cpp files together doesn't increase incremental compilation time significantly over simply building each .cpp file in isolation.
You're thinking of "MNG", which is a different PNG-related animation format than the simpler "animated PNG". MNG was removed, but animated PNG is still supported in Firefox and indeed is used in its UI.
Shipping new Firefox code that parses untrusted data from the network normally requires security view, and a new fuzzer for the data format in question is usually written. There's lots of process and infrastructure around this sort of thing at Mozilla. This is sometimes handled in a separate bug from the one implementing the feature. I'm surprised that the corresponding security review bug isn't linked from the FLAX bug, though, which may be an oversight. (Or it may be the non-public blocking bug, and it's non-public because an issue was found.)