Those are all essentially infrastructure projects that (one way or another) add value to end user products that (not always, but quite often) are monetised somehow and in turn contribute or donate back, because they can. Firefox OTOH is an end user product itself, quite a different case.
Note that I'm no fan of the Pocket integration at all - to my knowledge there isn't even any sort of bundling deal or other kind of monetisation involved. I don't understand why Mozilla does this.
I don't agree with you. Firefox is an infrastructure product; it's how end users interact with actual products they care about (e.g., Facebook, Netflix, etc). Few people fire up a web browser without the intention of consuming third-party (non-Mozilla) content, just like few people boot a linux system just to watch the kernel run.
You are welcome to draw your own conclusions about the viability of constructing a modern browser engine on donated time from a handful of Facebook and Netflix engineers.
OTOH, there's the counter argument that if search engines and OS vendors are willing to pay for browsers, why should Facebook and/or Netflix spend any money? It seems to be the classic business case problem of, "well everyone else is paying for it already, and as a result we won't have much affect".
I do, of course, realise that FB and Netflix very much have an interest in the web platform, and could undoubtedly influence it more if they were willing to contribute code. That said, it's probably worthwhile to point out to those who don't know that both FB and Netflix are W3C members, and have several people who contribute heavily to specs.
With very few exceptions, open source projects (particularly infrastructure projects that add value to end-user projects) are not getting financial contributions from the companies that monetize them.
Just look at GnuPG, OpenSSH, or OpenBSD. These are projects that produce some really essential infrastructure that runs the modern web. This software has been in use at companies like Amazon, Facebook, Google, IBM, etc for decades. They have received almost no support whatsover. Werner Koch (of GPG fame) was so broke and desperate that he considered getting a corporate job. Theo de Raadt tried to get support from any of the hardware vendors that used OpenSSH in their products. He eventually got a laptop from IBM after pestering them for a year. I doubt busybox or mksh get much in the way of support from Google or Android hardware manufacturers.
I would hazard a guess that Firefox is better-funded than most open source "infrastructure" products.
Note that I'm no fan of the Pocket integration at all - to my knowledge there isn't even any sort of bundling deal or other kind of monetisation involved. I don't understand why Mozilla does this.