> okay, but does mozilla have a good reason for not supporting that stuff other than "we have limited resources to implement these things"?

That depends on what you are referring to. No there is not a one size fits all answer. For example, Chrome has implemented Filesystem API that Mozilla is still debating on because they see it as a security issue. You can agree or disagree but there reason is still something other than "we don't have the resources to do it"

    For example, Chrome has implemented Filesystem API 
    that Mozilla is still debating on

This also highlights the vastly differing goals of the various parties.

Google explicitly wants two major things from Chrome.

One, they obviously want to track as much personal information as they possibly can, because they are an ad company.

Two, they want "the web" to essentially be a full OS replacement, with filesystem access etc. Because Microsoft is one of their primary rivals (or frenemies, if you will) and they can't leave themselves to the whims of others' platforms. They need their own platform.

These goals are... well, let's say divergent (to put it mildly) from what "the web" means to others. HTTP was originally supposed to be a human-readable way to publish and link information, not an OS replacement, and certainly not a PII-siphoning tool.

And yet, some folks still default to simply assuming whatever Google decides for the web is right, simply because they seem to be moving the fastest.

Yeah, they're usually moving the fastest, but people should think about where they're heading and why.

I feel like each feature has it's own story (and it's own party to blame, when some browsers support it but not others)

If FF is lagging behind Safari on a particular CSS feature that certainly points to FF being behind the curve.

Sometimes it's FF or Safari simply being slower than Google. Sometimes it's a matter of the Chrome team creating an implementation of feature XYZ and getting it minted into the standard so of course they have the only implementation for a while.

Sometimes the FF and Safari teams have specific objections to a feature, often because unlike Google they actually consider user privacy a core part of their mission. Although, of course, with CSS features... that's not gonna be a privacy thing.