Mozilla posted an update:

Update at 10:20 pm ET: Mozilla has since announced a change to the license language to address user complaints. It now says, “You give Mozilla the rights necessary to operate Firefox. This includes processing your data as we describe in the Firefox Privacy Notice. It also includes a nonexclusive, royalty-free, worldwide license for the purpose of doing as you request with the content you input in Firefox. This does not give Mozilla any ownership in that content.”