And why? It has nothing to do with Firefox.

I don't know about the Adobe case because I don't use Adobe web services. But it certainly could be Firefox's fault. For a long time I was a regular heavy user of a site that made extensive use of a particular CSS property that just was not implemented on Firefox. For years it just couldn't do the necessary behaviour for that site to work.

I don't use the site anymore, and it looks like Firefox has eventually gotten around to implementing it so it might work. But the point stands that a site not supporting Firefox could be Firefox's fault.

Personally, I'm of the opinion that a unified renderer is a good thing. Having all browsers be chromium based would make developers' jobs easier and would in turn provide users with a better experience. The individual projects like Edge, Vivaldi, and Brave can and should choose not to implement shitty things that Google is doing like Manifest V3 and Web Integrity API, without needing to have their own entire rendering engine like Firefox does.