Netscape 4 versus IE5 or 6 was pretty bad. They got stalled a few years while rewriting their browser. Firefox was better than IE almost from the very first alpha tests.
firefox was initially named phoenix and it was made as a workaround the performances issue of mozilla suite by removing everything unneeded to run a browser. Except mozilla suite fixed its performance issues way before phoenix was ready for prime time.
Netscape 6 was the "new one", with Gecko, XUL and stuff.
Firefox itself used to be an alternative - the main browser was Seamonkey, that included mail client, chat client and kitchen sink, just like the original Netscape did. Firefox started later as a lightweight alternative, just the browser.
