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I have reset FireFox a dozen times as the default browser. It lasts for a couple days, then the message "Firefox is not your default browser" Click on set as default, etc. Set it as default via Control Panel, etc. etc. I'm wondering whether CCleaner defeats the default assignment,... or maybe something else such as scanning for virus's. Ideas? I have started using Chrome, and like it, but I prefer FF's home page.

Did this start when you installed Chrome. If you make Chrome the Default browser on initial installation, it hangs onto that like a Velcro covered leach. And of course, the default Installation setting is to make Chrome the " Default Browser ". Gotta watch them danged little check boxes....

Here's are several ways to correct the issue. Note: This is for Windows 7, but same procedures for Win 8 and 10.

https://superuser.com/questions/970837/how-to-prevent-google-chrome-from-changing-the-default-browser-to-itself-in-wind