Even among Firefox users, only about 41% have add-ons installed in the first place: https://data.firefox.com/dashboard/usage-behavior So 59% of current Firefox users could switch to Chrome without being affected by the Manifest V3 change at all.
I don't know how many Chrome users use add-ons that would become less useful after the change, so it'll definitely be interesting to see whether Firefox's numbers will show a sudden uptick at some point.