Earlier quoted context omitted.
The Firefox rewrite was a success by all measures. Spolsky is wrong on this one, at least with the claims of generality.
Except for one measure: Netscape died as a company. The huge rewrite contributed to killing it. If you don't ship a product (for like 4-6 years?) you're gonna die. Mozilla originally chose the name phoenix, (then firebird to avoid trademark problems, then finally firefox) was chosen because it was a phoenix rising from Netscape's ashes. Its major innovation: It was 'blazing fast' when compared to ie 5.5 / 6. Tabbed b…
The successor of Netscape Communicator was Mozilla (IIRC it was just called that, later renamed Mozilla SeaMonkey), and the successor of Netscape Navigator was Mozilla Phoenix (later renamed Mozilla Firebird and eventually Mozilla Firefox). Firefox and Thunderbird were once again separate clients.
Mozilla was still considered bloated, but Phoenix was far less bloated which is nice on lower RAM machines, and allowed the start of Web 2.0. It was also the return of doing one thing and doing it right: browsing the WWW. As Netscape Communicator (unlike its predecessor, Netscape Navigator) came with a Usenet client and e-mail client.
Later in development, addons became a thing, and you could add features which were previously part of Netscape Communicator such as calendar, HTML editor, etc. You can also add such features with addons to Mozilla Thunderbird.
Then Google Chrome happened, and people switched to that, but I'm not entirely sure why.