It's a good reason IMO. MS browsers push MS services and Google does the same with Chrome. For example, I have to be logged in with my G account to download an extension for my browser. I am struggling to see the connection between the two.
Finally, google links everything together (as we saw with the resold Pixel debacle), so why would I want to increase the risk of breaking Google TOS by using their browser.
Firefox pushes Mozilla services, such as their "Sync". They've also pushed commercial services, such as "Pocket", over user objections.[1] And, of course, they were paid to make Yahoo (of all things) the default search engine.
They don't really push Sync. Most people are unaware of it.
I disagree with many of the concerns over Pocket as they are alarmist and overblown. The Pocket code is MPLv2 (open-source) and it isn't "spyware". This is trivially verified by looking at the code.
Mozilla used to have a partnership with Google that accounted for 85% to 91% of their revenue. Mozilla needs funding to survive, and Yahoo! provides some of that funding.
It is a false equivalence to suggest that the way Microsoft or Google pushes their services is anything like Mozilla's actions. Mozilla has always pushed for an open web and strong privacy protections. Google only implemented Do Not Track after Firefox did. Mozilla just released an ad-blocking browser for iOS to go along with their ad-blocking add-on. Comments like these only push that false equivalence.
Mozilla's Firefox Sync server code is open source and even packaged so you can run your own Firefox Sync server, if you are concerned about privacy or Mozilla no longer supporting the service:
Finally, google links everything together (as we saw with the resold Pixel debacle), so why would I want to increase the risk of breaking Google TOS by using their browser.