My current excuse? Android (the only current realistic alternative; I don't have the time nor care to fiddle with technical details on a device that I need to be a reliable tool. I'm not afraid of going deep on something less standard, Linux has been my daily driven OS for literally decades) is trash and untrustworthy in my experience. Browser extensions alone are not nearly enough to tip the scale for me to go back to it. As mentioned I use something that provides extension support already (from a browser that has far fewer developers working on it!), which is why I find it stupid that Mozilla won't figure out a way to do it themselves.

Where do I buy this device that does let me run any software I want?

Most Android phones don't even let users unlock the bootloader anymore. And if you do unlock the bootloader, now Google Pay doesn't work, among other things. I don't really feel like being in control of what software I use when the software I actually want refuses to run. F-Droid homepage is also concerning.

Linux phones barely handle the basic functions.

1. Not wanting to.

2. Caring about other aspects of the device.

I don't personally agree with Apple's decisions, but you "vote with wallet" people need to have at least some modicum of empathy before you start demanding that people have a lot for you and join your cause.

As a mobile device? There are like 3 of them on the market right now and you'll run into huge headaches at every turn if you try and use them.