Hi Tommaso, when you select View in Browser in the Microsoft Outlook desktop application, Microsoft extracts the message to a temp folder and converts it to MHTML format, with a name similar to email.mht. Firefox cannot natively read .mht files, unfortunately, so it is not an option to view them in Firefox unless there is a new add-on that can handle them.

To switch your handler for .mht files back to IE, you could try this:

In the Windows 10 system search box, type assoc and click the link to the control panel "Choose a default app for each type of file"

After that eventually loads, slowly, like watching water boil, scroll down to .mht and re-associate it with Internet Explorer. If it's not listed, you might need to specify it as a "feature" you want on your Windows 10. See: https://support.microsoft.com/en-us/help/4026136/windows-10-use-internet-explorer

Thank you.

So, if I understand, I can open links but I cannot open the email itself, as Outlook offers (image in my previous message). Is this summary correct?

Again, thank you for your time and expertise.

Yes, if there's an HTTP or HTTPS link in the message, it will go to your default browser automatically, which can be Firefox.

Only Outlook's own "View in Browser" feature uses .mht files, which Firefox cannot display.

I assume that Outlook opens a video player because it assumes that the video player is the closest thing to IE (I disinstalled IE).

Again, thank you.

Okay, you can re-add IE 11 since Office needs it for this feature.

Never MS! I'd rather suffer than make room for more MS on my watch!

Mozilla forever!