1. see the link and UNcheck Always check to see if Firefox is the default browser on startup, click OK to save it.

How to make Firefox the default browser

2.also see the link and UNcheck Mozilla Firefox

https://support.mozilla.org/en-US/kb/Setting-Firefox-default-browser-does-not-work#w_windows-xp

be sure to check another browser as default browser

thank you

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You need to set another browser as the default browser if you no longer want Firefox to be the default browser.

• Control Panel > Add/Remove Programs > "Set Program Access And Defaults" (on the left)

• Open: Custom (click arrow on the right)
• Select "Internet Explorer" under "Choose a default Web Browser"
• Click OK