LineageOS[1] exists, and it may as well be a non-GNU mobile distribution of Linux with support for Android apps like Firefox.

[1] https://lineageos.org/

The "Linux" kernel even in something like LineageOS is heavily forked and in practically every case relies on userspace binary blobs for critically-important functionality. It's a far cry from what you can run on PC's, although postmarketOS is moving towards that goal. BTW it's not like Android itself is to blame for this, since other varieties of embedded Linux (by and large) are the same deal; the buck stops with SoC- and embedded-hardware manufacturers.

I've given up on custom ROMs. There's always only an extremely limited set of devices that are supported, and those devices are EOL rather too quickly, forcing us to use custom builds from someone not directly affiliated with the project.

To be clear, not one of the Android devices I've purchased have ever had official support. I'm using a Samsung a8 at the moment.

I hope the custom rom community finds a better mechanism for providing better device coverage.

Custom ROMs are rarely developed for Samsung devices these days because Samsung locks the bootloader and makes everything extremely difficult. Buy a device that is more developer-friendly like a Nexus, Pixel or OnePlus and you'll see a much larger selection of custom ROMs.

The last Nexus was released 4 years ago. The two pixel models and the OnePlus 7 make 3 choices of currently manufactured models.

Are you aware of a current model phone that is officially supported by lineageos?

lineage always supports google devices (pixel)

Their docs only list the first generation Pixel:

https://wiki.lineageos.org/devices/#google

The Nexus 5X isn't supported (yet?) by the latest version of LineageOS (16).

If there was a viable competitor, I would consider using it. There isn't.

postmarketos is beginning to support cellular capabilities. You may not have to wait a whole lot longer.

There is. You just don't want to spend the money or use the phones/tablets from said competitor.

I'm personally a big fan of iOS, but coldpie's (and others in this thread) needs seem to be in third party browser capability that doesn't exist in iOS so with that, I wouldn't recommend it to them. I don't think it is a matter of spending money on competitor's devices so much as competitor's devices don't meet their needs.

It'd be acceptable if we could run code on it, as root, like we actually own it.

iOS doesn't allow anywhere near the level of customization that Android does, nor does it allow 3rd party browser engines at all, however there's Sailfish OS and maybe the Librem 5 soon.

iOS doesn't allow you to use other browser engines.

iOS is not a competitor for my needs.