[Millennium]

As long as all of the source is available under FOSS licenses, yes. Free as in speech, not free as in beer, remember?

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?

[jasonhansel]

Ideally, Mozilla would let us pay to use their servers, while also providing an OSS self-hosted version with equivalent functionality.

(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.)

[Crinus]

MPL does allow closed source extensions, Netscape wanting to create a proprietary browser on top of Mozilla was the main reason MPL was created and they didn't use something like GPL.