You can do it but that doesn’t mean you should.

Throwing away a phone whose hardware may be perfectly working, but its manufacturer decided to be too cheap to use anything but walled-garden components, feels like an utter waste of resources.

(By walled-garden components, I mean chips whose suppliers make it difficult or impossible to develop an open-source driver.)

If you really need that old phone, use Firefox Mobile which has its own root store.

But good luck using it

This might be less of an option then you'd think. Android software is often compiled assuming the CPU has comparatively recent instruction set extensions. Older devices, especially lower-end devices (which are more likely to be stuck on older software and more likely to be owned by people without the resources to frequently upgrade), might lack those extensions, making the app unusable. This is not a hypothetical. I've experienced this with FF a couple years ago (and even spent a couple hours trying to figure out how to build from source, but ultimately gave up.)

FWIW, I use Firefox on a 10 year old phone with Android 5.1, and it runs just fine. The phone is built on an x86 Atom SoC, though.

https://www.intel.com/content/www/us/en/products/sku/75203/i...

What extensions other than NEON are you thinking of? Neon was made a CDD requirement for Android 6.0, released on 2015. Even before that it was almost ubiquitous. The last shipping devices in any volume missing NEON were Tegra 2 tablets from 2011.

That "almost nothing" is a typical blue collar monthly salary in my shitty town.