[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.