
Hi @Daniel Hitson,
If your computer is managed by your organization, then you are expected to have default browser changed according to the group policy if any. Please reach out to IT for more information.
If your computer is a personal one, make sure you haven't got Protect your default browser setting enabled. You can find this setting by navigating to edge://settings/defaultBrowser in Microsoft Edge. What's more, sometimes third-party anti-virus software can also change the default app settings. You can try disabling them if any for a test.
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Best Regards,
Shijie Li