Firefox is my default browser and has been for years. But today, opening it suddenly brings up a pop-up window asking me to make it the default. I click "Yes" and "Don't tell me this again", but the next time I launch Firefox I get the same window. The Windows "Set Default Programs" page also doesn't fix it. Neither does rebooting. I know Firefox is still operating as the default because when I close it and then open a web link shortcut on my desktop, it opens with Firefox. How can I get it to stop bugging me to choose it when it's already been chosen?

I'm running Firefox 35.0 on an updated Windows 8.