It's goal was "bug-for-bug compatability with IE" so that you could run Mozilla / Netscape on websites that weren't updated / weren't ever going to be updated.
KHTML / Konqueror was effectively "strict-mode-only" not caring (as much) if sites broke, but implementing things "as sanely as possible".
Firefox was Mozilla with a sane UI on top of it.
Mozilla UI was trash because goofballs in suits kept ruining it by pushing for "site-specific-themes paid for by advertisers" which caused the "chrome" to be incredibly buggy / slow / etc.
WebKit / Safari was Apple delicately picking up KHTML, making the "hard" decisions to implement some things poorly / hackily / different / more quickly than the "purist" open source KHTML volunteer developers had envisioned.
They actually handled it overall quite well, as opposed to their other forays into open-source land (kernel / darwin, cups, etc).
WebKit is excellent now because it didn't have to start with that bridge step, and had speed / correctness / isolation as a focus from the start. So even though WebKit is the current "leader", it owes a lot to Mozilla for doing the hard grunt-work that allowed it to take cover behind the big lumbering dinosaur and come out unscathed on the other side.
Either way according to Wikipedia, Don had forked KHTML/KJS in 2001 (which pre-dated the first public release of Phoenix by a year or so), so his choices were to hack SeaMonkey into something suitable or to start from somewhere else.
