Here's the actual quote about search:

Firefox by default sends search queries to your search provider to help you discover common phrases other people have searched for and improve your search experience if your selected search provider supports search suggestions... Learn more, including how to disable this feature...

...

If you enable "Improve the Firefox Suggest Experience," we and our partners may also receive your search queries.

So it sends search queries to get search suggestions. I didn't see it mentioned one way or the other, but I'm assuming Firefox doesn't send any personally identifiable information with it, though the server probably can track you somewhat with your IP address.

Sending queries to partners is optional.