https://www.mozilla.org/en-US/privacy/firefox/

If you use Google as your default search engine and you search, that obviously lands with Google. If you have type-suggestions enabled, typing input is sent to Google. (Also obvious, to me at least.)

Mozilla may also receive location-related keywords from your search (such as when you search for “Boston”) and share this with our partners to provide recommended and sponsored content. Where this occurs, Mozilla cannot associate the keyword search with an individual user once the search suggestion has been served and partners are never able to associate search suggestions with an individual user. You can remove this functionality at any time by turning off Sponsored Suggestions — more information on how to do this is available in the relevant Firefox Support page.

Turn off sponsored suggestions and locations you put in to search are not shared with sponsors/partners.

You may be able to opt into an enhanced search experience, which will result in Mozilla processing additional personal data, including your technical data, location and search data. Some of that information may be shared with our partners on a de-identified and/or aggregated basis. See our “Data and Firefox Suggest” blog post for more details.

Don’t opt in to enhanced search experience and they won’t share that with partners.

To serve relevant content and advertising on Firefox New Tab

You can opt out of having your data processed for personalization or advertising purposes by turning off “technical and interaction data” on Desktop and Mobile at any time.

Opt out of personalized advertising.

To provide AI Chatbots

If you set up to use an AI Chatbot third party service, what you type will be sent to them (obviously). Not to Mozilla either way.

To provide Review Checker, including serving sponsored content

Review Checker is a Firefox feature that helps you determine whether reviews are reliable when you shop online with sites like Amazon.com, BestBuy.com and Walmart.com. If you opt in to using Review Checker, Mozilla will process information about the website and the product identifier of the products you view using our privacy preserving technology called OHTTP.

Don’t opt in to Review Checker and they won’t share even in OHTTP form.

To market our services

Firefox also shares information with our marketing partners to measure and improve these campaigns; what information is specifically shared varies (depending on how you discovered Firefox and your operating system) but generally includes how you were referred to our download page and whether you actively use Firefox

From the support article:

If you arrive on the Firefox download page after clicking an ad on another website, you’ll have the option to share how you discovered Firefox with Mozilla’s marketing measurement partners.

Don’t approve sharing the data and it won’t be shared. (This is kinda outside of Firefox the software anyway. It’s about them measuring how users arrive at installing Firefox and their campaigns regarding that.)

---

If you install Firefox and disable sponsored content and choose a non-Google search engine, nothing is shared with Google or other partners outside of what you select and explicitly consent to/set up.

The relevant settings are under Settings -> Home (Sponsored shortcuts), Search, and Privacy & Security