There are plenty of distros of Linux that have fully removed binary blobs [1]. As far as I am aware, it wouldn't be allowed to fork Mathematica and remove all proprietary blobs.
Realistically though nobody uses those, and every project like that such as BSD gets used as a base for proprietary software. Do they even support TRIM?
Sure, but that doesn't really take away from my point. I think people are more willing to say that Linux is "open source" because they are allowed to fork it and make their own thing, embodying the spirit of FOSS.
It's perfectly valid to criticize the kernel maintainers for including binary blobs, it certainly annoys me, but it's not equivalent to what Wolfram does with Mathematica.
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I actually used Trisquel for about a year. It was fine.
> Is the ability to fork the essence of what makes a project open source?
Kind of? The ability to "do whatever you want" with the code more or less embodies the experience of FOSS.
> Why did you stop using it?
I got a job at Apple, and roughly three months after I started there that laptop broke and I got a discount on a Macbook. I know I could put Linux on there, and maybe at some point I will, but currently my school requires that we either have a Windows or Mac computer, and since I don't want to go about mucking with virtual machines, I'm sticking with macOS.
Not even drivers, AFAIK; it's firmware blobs that have to be loaded to the device in order for it to run, but nothing on the actual "host" computer side. So like, if you have a wifi card with blobs, you have to hand it a firmware blob to "boot" the card, but once it's up you can talk to it using a fully GPLv2 driver in Linux (which talks to the proprietary wireless firmware over PCIe or whatever).