Firefox displays nine boxes or "tiles" showing screenshots of the websites visited most often by the user.
Many users were concerned about Mozilla's plan announced earlier this year to add advertising to the boxes.
But the firm has assured users it is not going to turn Firefox "into a mess of logos sold to the highest bidder".
Johnathan Nightingale, vice president of Firefox, said in a blog post that many users had found the language of the initial announcement "hard to decipher" and were worried they would have no control over their browser.
"That's not going to happen. That's not who we are at Mozilla,"
He said the tests were designed to understand what the users find helpful and what parts they ignore or disable on the browser.
"These tests are not about revenue and none will be collected. Sponsorship would be the next stage once we are confident that we can deliver user value."
The company had previously assured users that while it will use information about their location to provide content relevant to them, no other user information would be "collected or considered".