I understand that speed is relevant, but focusing on that strategy does not really work when dinosaur-like extinction is around the corner.
Seriously, what is with the trend of assuming that anything anyone in a project does, it is assumed that it has been the main goal of the project and all other objectives have been abandoned? "Firefox's frontend designers are doing frontend design work instead of implementing WebUSB which would immediately reverse their declining user share trend and make billions of dollars worth of donations! What are they thinking?!!"
(I'm gonna get a lot of hate for this post, aren't I?)
1. Just because I'm a dev at Mozilla and I got support for buildcache landed in the Firefox repo doesn't make this a Mozilla project. I do this on my own time to help me be more efficient when I actually work.
2. I mainly do this because it's fun. Including the blogging.
3. Sccache is still the compiler cache we're officially developing at Mozilla
Safari is popular because it ships with iOS and macOS.
Edge (previously IE) is popular because it ships with Windows.
Chrome, however, is popular for several reasons. One reason is that it ships with Android and ChromeOS, but before that Google had a very aggressive multi-channel campaign where they pushed it with large banners on Google search (everyone used Google) and they made deals with Windows AV vendors so that when a user installed anti-virus on their computer Chrome was automatically installed and made the default browser. Another reason is that Google has consistently develeoped Chrome together with their web services, so things like search, maps, gmail, docs etc tend to work best in Chrome.
The only default channels that Firefox has, that I'm aware of, are verious Linux distros, and they have a pretty thin market slice.