Honestly, a very impressive move. Makes me way more confident in the trajectory of the company and I'm happy to have been a visionary user for multiple years.
I wonder, though, just how much of Proton A.G. does the foundation now own? They say it's the largest shareholder, but they didn't say "majority shareholder".
AFAIK, the main difference is that Firefox's process isolation on Linux specifically is incomplete. They're working on fixing that.