I actually think Firefox OS would be a lot more viable if they tried to make it today. A lot of things are webapps or JavaScript apps, so app support might have been less of an issue.
There's also blockchain which exploded, leading to a lot more developers with good funding willing to work on and/or donate to efforts like FirefoxOS.
Mozilla is exactly the company I most trust to guide the software that is going on my phone, and the browser I use. I'm not sure they would make it today even if they tried, the management and the goals of the project were what drove it into the ground IMO. They might have done better if they focused on the high margin cell phone market -- grab the blackberry and privacy conscious market who have money to blow.
At least we got a faster/better FF out of it -- then again so many sites are chrome optimized (and don't necessarily even test FF) that I'm a little worried.
Android is open source but doesn't feel like it in practice (especially if you buy a vendor build)... And it's surprisingly hard to find a vendor that will just give you a phone that can be easily wiped to put stock android on.
There's also blockchain which exploded, leading to a lot more developers with good funding willing to work on and/or donate to efforts like FirefoxOS.
Hardware support, on the other hand...