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
That's always the con of revenue streams, they impede Mozillas independence and ability act on behalf of users.
It's not ideal though. A large fraction of the web blocks access from Tor relays, or makes you jump through extra hoops, like completing onerous captchas. It's not a great experience.
Tor hides the source of your communication, and evades filtering. It does not protect the contents of your communications from eavesdropping. It's trivial to set up Tor exits to log traffic, and people do.
Grabbing a page from my website over tor and over normal network:
curl http://mywebsite
curl https://mywebsite
curl --proxy socks://127.0.0.1:9050 http://mywebsite
curl --proxy socks://127.0.0.1:9050 https://mywebsite
Results in these times (tor times depend on selected circuit): 0.028s / http
0.063s / https
0.394s / tor http
1.079s / tor https
If I killall -HUP tor (force changes circuit): 0.302s / tor http
0.598s / tor https