Thanks for working on such a great project. I've seriously hacked on the codebase about a decade ago and recall what a minefield it can be!
I'm not going to purchase a service I'll never use - I'm also philosophically opposed to centralised 3rd party VPN endpoints no matter who operates them. It's not a "Private" network, it's simply a proxy to side-step (most) ISP level interference or geo-fencing by services.
But if Mozilla can re-sell the Mullvad VPN service then sell me a unique personalised copy of Firefox that has my name in the Help>About* dialog! No messing about with donations then :)
*actually just read the string from a (possibly downloaded) prefs.js/user.js key.