Hi

I would advise treading very carefully through the Windows registry as much damage can be done by accident.

Looking at those registry keys, they appear to be governing the default app for various file types.

To check file handling in Firefox, this support article should help:

https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

Looking more widely at how Windows handles different files, this article may help:

https://support.microsoft.com/en-us/windows/change-default-programs-in-windows-e5d82cad-17d1-c53b-3505-f10a32e1894d

Let's discuss Outlook.

For links in the body of HTML messages, having Firefox set as your default browser for HTTP and HTTPS URLs should be sufficient for these. If you type protocol in the Windows search box, Windows should provide "Choose a default app for each protocol" as the top suggestion. With that panel, you can check HTTP and HTTPS.

Some Outlook messages have a special link in the heading area to convert the message to a web page. This is not a link inside the message body, it is a feature of Outlook that renders the page as an MHTML (or .mht) file. Firefox doesn't handle these, so it's normal for those to open in another browser. Don't assign Firefox to handle that file extension since it will not be able to render it.

Regarding the Registry keys:

FTP - Firefox discontinued its built-in FTP client, so don't change that one to Firefox.

HTMLFILE - If you type file type in the Windows search box, Windows should provide "Choose a default app for each type of file" as the top suggestion. With that panel, you can check .htm and .html (not sure if there is any other safe way to set this to Firefox).

Thanks to jscher2000 for the suggestions but these are already set to "Firefox" in my system. Maybe I haven't explained my issue properly but I will try to expand: When I run a program called CCleaner to clean up unused Registry entries I always get the suggestion to delete the unused Registry entries as shown in my original post. This seems logical because I have Firefox set as the default browser. However, if I do go ahead and delete the registry entries that show "C:\Program Files\Internet Explorer\iexplore.exe" %1" it then results in all embedded links in Outlook emails or Excel documents becoming broken and I get the error message: "your organisations policies are preventing us from completing this action for you. For more info please contact your help desk." This is a common problem found by searches on the web but all suggested resolutions do not fix the problem for me. In simplistic terms the question for me is: if I have elected and set Firefox as the default browser why do I need any reference to I.E in the registry and why doesn't Firefox open the links when I delete the I.E. registry entries?

I assume you restored the deleted keys.

Do links in Outlook open in Firefox the way I described that they should? If so, then Windows is working the way Microsoft designed it even if it does not look logical under the covers.

In simplistic terms the question for me is: if I have elected and set Firefox as the default browser why do I need any reference to I.E in the registry and why doesn't Firefox open the links when I delete the I.E. registry entries?

I suggest asking Microsoft that question.

Yes they do open BUT ONLY IF I leave the Registry entries relating to IEXPLORER.EXE in place. As soon as I delete those registry entries the embedded links become broken despite the fact that: (a) .HTMFILE is set to htmfile (b) .HTMLFILE is set htmlfile (c) Protocols are set to Firefox (d) Default browser is set to Firefox (e) Internet Explorer is not active (i.e. unchecked) under Windows Features in Programs and Features

So my question remains: (i) why do I need any reference to I.E. in the Registry; and (ii) why doesn't Firefox open the embedded files after I delete the Internet Explorer Registry entries?, and (iii) why does another app (i.e. CCleaner) when run suggest that the I.E. registry entries are unused?

I've been grappling for this issue for about 6 months now and every suggested fix doesn't not work. The only thing that works is to leave the I.E registry entries in place but the puzzle for me remains why are I.E registry values required when I don't use it; don't want to use it and it is not installed??

So my question remains: (i) why do I need any reference to I.E. in the Registry; and (ii) why doesn't Firefox open the embedded files after I delete the Internet Explorer Registry entries?, and (iii) why does another app (i.e. CCleaner) when run suggest that the I.E. registry entries are unused?

Regarding (i) and (ii):

Firefox doesn't know what you click outside of Firefox until Windows executes a command line with the URL or file name to load it in Firefox. Therefore, this is a Windows question for Microsoft, not a Firefox question.

(To some extent, it could also be an Office question because historically Office is uniquely subject to browser confusion. For example: https://www.slipstick.com/problems/this-operation-has-been-cancelled-due-to-restrictions/. But that is also for Microsoft to explain.)

Regarding (iii):

This is a question for the maker of CCleaner.

Thanks for trying your best to assist with my issue, I appreciate that you have taken time time consider and respond. However, I am not sure that you grasped the fundemental issue which simply put was: why the need for any registry entries relating Internet Explorer when it wasn't installed or being used?

HKCR\ftp\shell\open "C:\Program Files\Internet Explorer\iexplore.exe" %1 HKCR\htmlfile\shell\open "C:\Program Files\Internet Explorer\iexplore.exe" %1 HKCR\VisioViewer.Viewer\shell\open "%ProgramFiles%\Internet Explorer\iexplore.exe" -nohome

I took the bull by the horns and have replaced each of the above I.E. Registry values with: "C:\Program Files\Mozilla Firefox\firefox.exe" -osint -url "%1" Rebooted and found that all embedded links in Outlook Mail and Excel are working as expected.

This issue can now be marked as Solved. Thanks again for your efforts.