Assuming HN readers are creating a bunch of products people use every day:

I think at some point Chrome became significantly better than Firefox and did a better job listening to users, so power-users (developers) started switching, once they switched, they primarily developed for the browser they use themselves (it's just more convenient when you get a link to look at to open the inspector in the browser that is set as system standard), so support improved further on that platform, so market share would constantly rise. This in turn leads to more devs reducing their focus on Firefox, so users turn away when a site looks or functions worse.

Now, Firefox has caught up on a bunch of things, yet Google's market share is huge both in browsing and advertising. It would be good for more developers to actively support the strongest alternative platform, because this in turn might affect adoption of that browser, even if it's just small numbers (every site that doesn't work might lead to a user switching to Chrome). It would keep them at least a little more safe from a monopoly that might start imposing policies that are bad for their business, the same way app stores are causing issues because they are the only way to access mobile phones (outside of web app - which is the point of the whole discussion)

Edit: note there's probably many reasons - this is just one way of looking at it.

I mean, it’s the same story as Apple and the Mac. Consumers ask their pro/techie friend what they use, and do that. People buy MacBook Pros, people use Chrome. When pros say “I won’t use Firefox because this little feature doesn’t work exactly like Chrome”, their mom doesn’t either.

MacBook Pro is a tiny fraction of laptop user base.