If you are comfortable editing the registry, you could try the solution in this older thread:

https://support.mozilla.org/questions/1242740

Thank You for Your quick reply. This is exactly what is was looking for. Yet I don't know why it didn't turn up on search. I've clicked through 8-10 pages.

I've edited the registry without problems, but now the links in discord won't open anywhere and even windows thinks that FF is not my default browser :)

Anyways, I will probably have to reboot the machine for the changes to take effect, which I can not do right now. I will get back here tomorrow.

Thank You.

It shows up a system reboot wasn't necessary. The changes are immediate. I've tried various mix of single and even two command line options of the "-url %1", "-private-window %1" and "-private". It doesn't seem to work. Only time the win10 opens links per click is with "-url %1", that which was there before tampering :)

Hmm, now that I look in my own registry, I see there are numerous entries with program IDs like

HKEY_CLASSES_ROOT\FirefoxURL-308046B0AF4A39CB

Just changing -url to -private-window doesn't work currently for me, but if I also delete the -osint then it works:

"C:\Program Files\Mozilla Firefox\firefox.exe" -private-window "%1"

I don't know whether -osint makes any practical difference, so I don't know whether that is a bad idea.

-osint "Tells the application that it is being launched by the OS shell. This should not be specified unless the caller provides all of the functionality provided by the OS shell when launching the application (bug 384384)."

I don't know what exactly is covered behind that, but I always start FF right after I login. So FF only opens new tabs without going through a new start.

It works the way You described and I will try it out for a certain period of time and see how that behaves.

Thank You :)

"Private Browsing" should be entered by default whenever FF creates a new tab from a link in an email or any file outside FF.

There should be a user option in "Privacy" to DE-ACTIVATE opening external-to-firefox links in a "Private Window", thereby reverting back to today's ordinary, regular tabs or windows.

The single biggest privacy exploiters are, with few exceptions, the senders of commercial emails. Meanwhile... Thanks for the regedit patch.

The single biggest privacy exploiters are, with few exceptions, the senders of commercial emails.

That's a bit of a special case, because the parameters in those tracking URLs tell the server "who" clicked the link (well, whose message was the source of the link that got clicked). Tough problem.

Sadly some things can't be avoided, like the hash in the url that identifies your mail box.

The registry edit works and after a month I did not discover any weird behavior.

But after every FF update, I have to go and apply the registry change again, because it gets revered back by the auto update :)