Let's start with; Open Windows Internet Explorer. Have it declared as the default browser, But turn off the option to check this when the browser opens (If you want).

Now do the same thing using Firefox.

https://support.mozilla.org/en-US/kb/how-make-web-links-open-firefox-default

https://support.mozilla.org/en-US/kb/setting-firefox-default-browser-does-not-work

If there's still a problem, Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Here's another thought:

Firefox 67 changed how external links and locally saved documents are handled: instead of opening in whatever Firefox profile is currently running, they will open in the default profile. So possibly your regular profile and the one marked as default might be out of alignment.

Next time you use an external link and open in the empty window, try this:

Profile Manager Page

Type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least two profiles. (Each profile has two folders and one or more buttons. Do not use any Remove buttons!)

The profile that Firefox is currently using will have this:

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

The other running profile will have this longer text:

This profile is in use in another application and it cannot be deleted.

Hopefully that profile has a button labeled Set as default profile you can click to align the external link behavior with your preferred settings.

Any luck?

Thank you all for your helpful suggestions. They are all helpful in solving this issue.

Setting the default browser away to other browser and back revealed that my OS sees 2 Firefoxes. The last update left with a parallel install of 67 and 67.2 . So I removed the 67 version and then set Firefox as default browser - now the OS showed only a single Firefox available for link opening.

Works as ever. Thx again

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.