hi, firefox sync is making use of the password manager infrastructure to authenticate your account with the server & since sync fires up each time you start the browser you will see this prompt.

I have the same problem but every time I fill in my master password the pop-up stays coming back, it won't disappear.

Hi Annemarjo

In case of issues with Sync you can try this:

Disconnect from Sync and Reconnect to Sync again. You can disconnect from Sync by clicking the e-mail address in the "3-bar" Firefox menu button drop-down list or go to "Options/Preferences > Sync".

Delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave). Remove existing chrome://FirefoxAccounts entry in the Firefox Password Manager.

If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox