Hi rs_hh, sorry to hear about this problem. Firefox now opens external links in your default Firefox profile. If you start Firefox with a shortcut that uses a different profile, this can lead to the situation you describe where the links open in a window that doesn't have your usual settings.

If you haven't already, could you close out of Firefox and do a normal Windows shutdown and restart to complete any pending updates and release any file locks.

Then please take a look here:

Profile Manager Page

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least one profile and could list many. Each will show links to a pair of folders; please ignore those for now.

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

Actually, please do not delete anything here, don't use any Remove buttons.

Each profile has a set of buttons, and the non-default profiles should one to Set as default profile. Are you able to assign that designation to your preferred profile, or do you only see one profile here anyway?

Thanks for getting back to me so quickly!

Here is what the output of "about:profiles" in my FF 67 looks like. As you can see, there is only one profile which doesn't have the "This is the profile in use and it cannot be deleted." phrase with it.

The FF 67 I use links to that profile file as well as the new window that opens when I click an e-mail link in Thunderbird. But the window my mail links open in has neither add-ons nor other customizations.

I maybe should have been more elaborate on my FF setup: The FF 67 I use for web browsing (which has all my customizations) is a portable version stored on an encrypted partition (D:\, not C:\). On the same partition there is another, earlier portable version (FF 47) which I need to run a special add-on that is not available for Quantum. I use that on rare occasions when I need that special add-on. The version which opens on clicking a link in a Thunderbird e-mail is also FF 67, the one I use for browsing, but apparently running on an empty profile.

Okay, I'm slightly confused about what versions of Firefox are installed and running.

When the link launches from Thunderbird, what version of Firefox is it running in -- it's 67? You can use the Help > Troubleshooting Information page to check that without triggering an update. (Tap the Alt or F10 key to show the menu bar if needed.) I'm not sure whether the page distinguishes regular installs from Portable.

The Firefox 67 component that deals with selecting the profile may not know about your "Portable" profile because there would be no reason for Portable to add it to the Profiles.ini (or installs.ini) file in the default location. Maybe adding it yourself would work to direct external links to that profile, but I have not tried to set up any experiments with Portable, so I don't know whether that would work.

You could keep an eye on their forum for possible workarounds:

https://portableapps.com/forums/support/firefox_portable

This might be useful, I think it describe how to change the association for external links (HTTP or HTTPS) to Firefox Portable so you don't have to rely on Firefox to decide what profile to use:

Firefox 67+ launches with default profile when opening links from other applications | PortableApps.com

The discussion on the PortableApps forum solved the problem for me. I chose editing the registry - which one of the contributors suggested - rather than using third party software, and it worked like a charm (didn't even need to restart Windows).

Many thanks from Germany, Ralf