Google Analytcs and ads are so widely that most of your browsing will be tracked regardless of browser, even regardless of your search engine (but the search engine is also quite major since they can track what you were trying to search for before jumping into the web).
One wonders why all the UI issues weren't solved long ago. Good that they're fixing them now, but what were they waiting for to decide to do it? (Rendering performance is a separate topic, that's been waiting on rust/servo.)
Assuming that one switches to Google as the default search engine (I'd love to know how many do - additionally I beleive Google is the default outside the US anyway), there's effectively no difference. Neither browser does anything special wrt privacy, the same resources and scripts are loaded in both. Safari are the only browser brave enough to try anythng more exciting in this area. (OK, Brave are also in this area, but they're not of much significance right now.)
Sure, Firefox's private browsing does offer tracking protection. But most "normal" users don't seem to use that. And some would argue there's little value in blocking tracking in a session that will be wiped anyway.
Sure, you can find the web client sources. How about the server, and the mobile apps? The website makes a big deal of them being open source after all ( https://protonmail.com/blog/protonmail-open-source/ ).
Google Analytcs and ads are so widely that most of your browsing will be tracked regardless of browser, even regardless of your search engine (but the search engine is also quite major since they can track what you were trying to search for before jumping into the web).
But there's also a privacy policy explaining what chrome actually sends: https://www.google.com/intl/en/chrome/browser/privacy/