[r3bl]

That's already happening.

Tor and Firefox are working together to make Tor network the default within private browsing mode. A number of privacy-related patches from Tor have already landed in Firefox (example: Firefox now has first-party isolation). It's a slow progress, but it's on its way.

If you're interested about more, the project is called Project Fusion: https://wiki.mozilla.org/Security/Fusion

There was a discussion here a few months ago: https://news.ycombinator.com/item?id=17205441

[IngoBlechschmid]

Awesome, thanks for bringing this to my attention! Good to hear that this is being worked upon.

[flukus]

I wouldn't be surprised to see this get dropped now that it's interfering with a revenue stream.

That's always the con of revenue streams, they impede Mozillas independence and ability act on behalf of users.