You need to change this in Windows default programs list.

Open Firefox. Type about:preferences#general<enter> in the address bar. Turn off Always Check Firefox Default Browser.

Open Windows Internet Explorer (or whatever). Have it declared as the default browser,

The "Always Check Firefox Default Browser" won't check!

I think if Firefox is not your default browser, then the "always check" box can be unchecked. Anyway, if that doesn't work, another way to modify the setting is as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste shell and pause while the list is filtered

(3) Double-click the browser.shell.checkDefaultBrowser preference to switch the value from true to false

FredMcD said

Turn off Always Check Firefox Default Browser.

Which browser is currently the default browser?

Firefox

rjackson4016 said

cor-el said

Which browser is currently the default browser?

Firefox

As noted earlier, Firefox can't make a different browser your default. You can either trigger the other browser to make itself the default, or use the Windows 7 "Default Programs" control panel.