[Discuss] Firefox and Thunderbird not behaving with conflicting info regarding default browser

[Edward]

Running Debian 10 (stable) with Trinity Desktop Environment (based on
KDE 3.5). Having some issues with Firefox and Thunderbird, in that they
do not seem to be playing nice with the system.

If I open an e-mail with Thunderbird and click a link, instead of
Firefox opening up the web page, Thunderbird erroneously and
automatically creates an account 'Blogs & News Feeds' and thinks the URL
is a feed. This does not happen all the time, but happens enough times
that it has now become an annoyance.

When I check the system for the default browser, it is giving me two
different answers:

~$ update-alternatives --config x-www-browser
There are 4 choices for the alternative x-www-browser (providing
/usr/bin/x-www-browser).

   Selection    Path                        Priority   Status
------------------------------------------------------------
* 0            /usr/bin/vivaldi-stable      200       auto mode
   1            /opt/trinity/bin/konqueror   100       manual mode
   2            /usr/bin/firefox-esr         70        manual mode
   3            /usr/bin/vivaldi-snapshot    0         manual mode
   4            /usr/bin/vivaldi-stable      200       manual mode

Press <enter> to keep the current choice[*], or type selection number:


:~$ xdg-settings get default-web-browser
firefox-esr.desktop

Firefox ESR is their Extended Service Release of the browser. Upon
checking the Firefox preferences, it is displaying that it is *not* the
default browser, even though xdg-settings clearly is saying it is.

When I launch Vivaldi stable and check the settings, that is telling me
that Vivaldi is also *not* the default web browser.

As Trinity is based on KDE, it also includes Konqueror, primarily as a
file manager, but obviously also as a web browser. I also have
PCManFM-Qt installed as a file manager, thinking that Konqueror could be
the cause of the issue, since something seems to be switching Firefox
from being the default browser, to not being the default. But when I
have used PCManFM as a file manager, I am still seeing this issue with
the browsers and Thunderbird.

Could there be some explanation as to what is going on?

Thanks in advance.

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