First about that famous Joel Spolsky post warning about software rewrites using Netscape as an example. Then the fact that Firefox have waited too much to create a multi-process browser which was really a needed rewrite to really catch up to Chrome.
If my memory doesnt fail me, Chrome was launched in 2008 and Firefox came up with the multi-process only 4 years later, in the end of 2012.
And then theres this rewrite of core pieces in Rust, that i'm sure was great for Rust language and community growth, but i dont think it was a good move for Firefox, because it take a lot of effort to stabilize the core pieces, specially while at the same time refactoring a compiler in the middle.
Rust turned out great, with a great performance, but what if it didn't?
Or maybe i'm being a half empty glass kind of person, and Firefox were actually turning out to the same faith, and at least a good tech, Rust came out of all this mess?