Is it still FOSS if it's paid?
Question is, will Mozilla actually stick to that? What the paid features also be open-source, at least to whatever extent the concept of source code applies?
(Firefox is available under the MPL, which is a sort of compromise between copyleft and MIT-style licenses. So it'd be difficult to make any new FF feature entirely closed source.)
Yes because there is nothing preventing you from selling free (libre) software. Commercial use of free software is actually encouraged and part of its success.
No because while you can make people pay for free software, you can't prevent redistribution. In practice, it means you can't force people to actually pay.
That's why selling free software usually means selling a service. Usually some form of support or online account.
Note that proprietary software based on free software is not free software. For example Chrome not free even though Chromium is. And if Mozilla releases paid version of Firefox with even a tiny bit of proprietary code, it stops being FOSS.