You have to get Firefox to handle MHT files. Noted at https://support.mozilla.org/en-US/questions/1152263
May have to install a Firefox extension.
I just had Windows 10 home and Microsoft office Pro 2016 installed on my computer. I have Firefox set as my default browser through the control panel. My problem is whenever I am in Outlook 2016 and click on the link in one of the messages it brings me up to Internet Explorer. What is also confusing is that if I have Firefox minimized on the taskbar the link will take me to the Firefox browser. If I don't have Firefox open when I click on the link in Outlook he comes up in Internet Explorer. I hope I haven't confused you too much. Thank you in advance for any help you may be able to offer me.

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