I can't think of a more effective way to kill Firefox. You'd give the fence-sitters a reason to switch, you'd get rid of all the volunteer contributors, and you'd get rid of everyone who uses and advocates for Firefox because it's free* software, which would probably be a significant portion of the remaining userbase. (I know it's technically possible to charge money for free* software, but it's impractical.)
Fence-sitters already switch in droves, Firefox lost 250M users in the last ten years. So keeping the status quo already clearly leads to demise.
And how can you claim you are free* when 90% of your revenue comes from your main competitor?
Maybe making a paid browser is what they need to become truly free* and reset to a user-centric product development culture, which could then actually make it an exciting product again (even if they initially lost 90% of users as a consequence, those would be lost with time anyhow).
I think you're underestimating the size of the demographic who care about FOSS, or who believe FOSS to be more trustworthy. It's not big globally, but I think it has a lot of overlap with people who use Firefox and people who care about online privacy.
There's no way that the intersection of people who care about online privacy, don't mind that Firefox is less web-compatible than a Chromium-fork, but also don't care at all about FOSS is big enough to sell a product that everyone else is providing for free, especially since you're not just saying goodbye to the Googlebux, you're also saying goodbye to part of your workforce (the volunteers,) free advertising (FOSS advocates and default installs on desktop Linux,) web devs (no one would pay to test pages on a niche proprietary engine,) extensions (most high-profile extension devs are FOSS advocates,) and most of the community. It wouldn't work.
What I am advocating for is making Firefox users also become its customers, vs current situation where user!=customer, and customer being Google. This obvious and giant conflict of interest has to be a major contributing factor to the decline of Firefox.
I am aware that Firefox may lose 90% of its user base if it switched to a paid model, but my point is it will do so eventually anyway - if things do not change radically.
So much this! Considering how much of my work and play time is spent in a browser, I would pay a hefty sum for a browser that met my needs and kept me safe from the data thieves. I'd even pay a subscription for this, where many current products do not deserve a sub model.
* as-in-freedom