The goal of most web developers is to make pages users can use, not get mired down in the never-ending browser wars.
As the smaller vendor, it's incumbent upon Mozilla to break it. Expecting individual devs to do it collectively when it really isn't in their selfish interests is waiting for a unicorn to appear.
That's kind of impossible to do for a smaller vendor without wider developer cooperation.
I remember Mozilla only started to breach IE's dominance once devs were so sick of IE that they installed Firefox on their mom's computer despite tons of sites being made for IE.
It's possible for something like it to happen again with Chrome, but less likely since Google's a lot smarter and not too lazy to implement latest tech, so it will sure take longer without some activism and evangelism from web devs.
> expecting individual devs to do it collectively when it really isn't in their selfish interests is waiting for a unicorn to appear
Selfishness is a lot more complicated thing than people give it credit for. A lot of 'selfless' acts could be alternatively described as selfish in that it makes one feel good. Free software developers already do a lot of work for the wider community, where probably it would be a lot easier to just use the proprietary, already feature-rich, counterpart than trying to develop a libre alternative. But the movement understands that long-term, having as much free software as possible is what will in the end help preserve general-purpose computing in the sea of silos. It takes some discipline, sure, but long-term it's actually in one's selfish self-interest.
Short-term, sure. Long-term it opens devs to Google's whims and makes the "open" web barely more open than Apple's AppStore.
But that's short-term vs long-term thinking and I can't deny most would prioritize the short-term. Here's hoping there's still enough idealists, even among web devs to avoid that fate and bring about for the web what GNU did for UNIX in the 80/90s.
The only space I can name off the top of my head where my open-source architectures have outstripped Windows and Mac in UX is virtual desktops.
Of course proprietary software has more funding to hire designers and such, but in terms of actual functionality, I'd contest your claim.
If you're talking developer user experience, it's not even a race. The FLOSS ecosystem has a landslide lead here. In fact the whole point of WSL is to try to keep devs on the Windows platform by bringing that experience to Windows more directly.
Customizability is orthogonal with out-of-the box UX, the original axis of comparison here. In fact, the two are often at odds.
The history of biological evolution shows that monocultures invariably fail catastrophically. Diversity is the main way to guard against unpredictable events of the future. Software is not exempt from these general rules I'd presume.