An earlier solution (https://support.mozilla.org/en-US/questions/1008285) shows how to get started. If the message goes away . safe mode, turn off all extensions, then restart. Turn them on again one by one until you find the offending extension. In my case it was extension "Mozilla Partners" (go figure). Disable and/or remove the extension. Remember each time you see the "Make Firefox your default browser" message, it sets the check box to "check every time". So you must un check the box each time it is displayed before closing the browser.