P.S. I think the problem always happens under these condition.

First any container tab add-on has to be installed, to make container tabs available. I've tested these: https://addons.mozilla.org/firefox/addon/multi-account-containers/ https://addons.mozilla.org/firefox/addon/cookie-autodelete/ https://addons.mozilla.org/firefox/addon/firefox-new-container-tab/

Then the same domain must currently NOT be opened in any other tab. Except in at least one tab which is a container tab.

Under this condition, Firefox seems to open links to this domain from external programs in the same container. Tested with Firefox-140.5.0esr and Firefox-145.0.2. Tested with newly created Firefox profiles.

This behavior may be quite useful for informed users. But without any explanation it's very irritating. Especially if the existing tab with the same domain in the same container is in another Firefox windows, outside of your current view.

Hacky workaround:

.

Create a local file: forward.html

   <script>location.href = location.hash.slice(1)</script>

.

.

Now you may call Firefox on this file instead of calling Firefox directly on the URL. This will open "forward.html" in a new tab, so Firefox won't use a container when creating the new tab. ("file://" is important to pass the #... part)

   /usr/bin/firefox file:///home/user/forward.html#http://example.net/

.

.

You may automate this on Linux with a Bash script: firefox-no-container.bash

   #!/bin/bash
   new_args=()
   for arg in "${@}"; do
       new_args+=("file:///home/user/forward.html#${arg}")
   done
   exec firefox "${new_args[@]}"