Do you mean this page? https://www.mozilla.org/firefox/53.0/whatsnew/ If you read the text above the form and in the form, you can see what this is about. IF YOU WANT MOZILLA TO TEXT A LINK TO YOUR MOBILE DEVICE for a mobile version of Firefox, then Mozilla needs your number. If not, just close the tab and carry on. If it was on some other page, what page was it -- can you find it in your history?