I think paying by mining is a good thing and I'm surprised how unpopular this view is. I don't particularly like ads and making a small payment or subscribing is a way too much hassle for a quick visit on a news site. I'd be totally ok with the site consuming up to 90% of my cpu while reading an article.
The problem is not mining to pay, IMO that's okay.
What is definitely not okay is doing so without consent or informing the user, this applies doubly on mobile devices with limited bandwidth and battery.
I think in spirit its a good thing. I do want to incentivize content producers to produce good content and recoup costs.
The reality of it seems far different. What stops a site from running all the miners at once? They'll likely pick what produces the most returns and may never audit shady networks. Then there's the notion of me getting what I paid for. What stops a site from getting more from me than the content is 'worth' and pocketing that change? Multiply that by every site.
We may or may not agree that ad networks and their tactics are to blame for the current situation. We're also dealing with sites that are either complicit by ignorance and have never audited their content, or they're complicit by straight malice and just need their money however they can get it.
As a user that seemingly can't trust any of the players, I feel like Mozilla has my back. As a web developer, I have a blog with zero ads with the notion that if I can't afford the $5 vps it runs on, maybe I shouldn't produce said content. That doesn't hold very much water for most useful content producers though because at some point the cost of infrastructure becomes to much to handle.
I think ultimately I'd feel better about a 'browser blessed' mining network or set of standards or something more altruistic that somewhat like Patreon, showed me what I earn and let me distribute it how I see fit. It would be more akin to tipping culture in Europe over the US though, where I gave above the usual service and not guilt me into helping them make ends meet.
It's great to hear that Mozilla continues to improve Firefox. I can only hope that they manage to avert a full Chrome-is-the-new-IE scenario, it's already pretty bad (can't even file taxes in FF only Chrome)