[valdask]

Probably because "you have to support it". If it does not work out of the box, but does work in Chrome, noone is going to bother to fix it. Same thing happened to IE - required additional code in order to work, and devs ignored it unless requested to do so.

So if FF made sure that everything is matching 1:1 (css, js), then it should just work... But sometimes as each browser decides to implement same thing using different naming.. Things break.

[Grumbledour]

As a primarily Firefox dev, in my experience, sticking to standard web technology, all I build in FF will 99% of the time work in chrome. So if building in Chrome will often not work in Firefox, this rather seems to be the developers fault? I get the impression, Chrome today is rather like the time of IE, where developers used a bunch of non-standard features (or in chromes case maybe to new ones) that then make the page unusable in other browsers.

I get that putting extra work in for special browsers is not great, but these days it should be not more work than using some vendor prefixes for css rules. If your page works in chrome but not Firefox, I would suspect it also wont work in edge, safari or opera. And that's on you as a developer, not on these browsers.