Does your option look like this?

If Windows10 did allow you to set Firefox as your default browser ...

In Firefox in Options > General un-tick the box under Startup that says Always check if Firefox is your default browser.

@the-edmeister, Thanks, i did do this already to stop the issue, all this is a fix doesnt really address the issue as to why its occurring new update for FF. WestEnd, yes its the same options and done as per pic. Thanks AGAIN guys

@CCTAT2, I don't have a issue when I do that. I think something else is has gone wrong if you can't get it to accept the change. I know FF does give you the option to set as default Browser but if Windows is changing it or blocking it this could be a Windows O/S problem.

In some scenarios after a browser update, the OS setting for the default browser fails to get updated. Change your default browser to another browser and then change it back.