Same with e.g. PostgreSQL.

But these are fundamentally different type of projects. Many businesses and products run on top of Linux and/or PostgreSQL. There is a very clear and obvious incentive to contribute, because that will help you run your business better.

With user-oriented software such as a browser, this is a lot less clear-cut. Organisations like Slack, or Etsy, or Dropbox: sure, they've contributed resources to stuff they use like Linux, PosgreSQL, PHP, Python, etc. But what do they get out of contributing to Firefox? Not so obvious.

I think this is one reason (among others) that Open Source has long been the norm in some fields oriented towards servers and programmers, and a lot less so in others.