I feel your pain, though I blame the websites more than the browsers. I can run full firefox on my OG pinephone as long as the sites I visit aren't running a pile of JS lunacy.
My Onyx Boox runs Firefox happily too, but for that and the pinephone I'd recommend trying out a Gemini browser (there are several on fdroid). It's lacking plenty, but has an old-web feel, no sites are slow, and you can handily use it on old devices.
> I feel your pain, though I blame the websites more than the browsers.
I blame the standards organizations.
I love the Boox devices as well. My Note Air 2 Plus is almost two years old, it is a terrific machine. I'd love to know what your use cases are. I mostly use it for taking notes and reading - including on the web - but I'm rather unhappy with most browsers on it including Firefox. For one thing, Firefox on Android has almost no keyboard support.
Oh yes, I think the web standards orgs are culpable (especially W3C) for not making it fast and simple to implement the features PHBs demand. This is surely in spite of the good people working on their behalf, and more a sad molochian consequence of regulatory capture big big adtech.
My Boox Color 7 was originally bought for terminal work and I connect it to my bluetooth keyboard and use tailscale and termux to make it a nice e-ink "dumb terminal". It's also nice for reading books on, and downloading said books (though I miss Kindle's email transfer method). I did try watching a YouTube video on it once, the result was almost watchable and far better than expected.
My Onyx Boox runs Firefox happily too, but for that and the pinephone I'd recommend trying out a Gemini browser (there are several on fdroid). It's lacking plenty, but has an old-web feel, no sites are slow, and you can handily use it on old devices.