What I have been able to determine, so far:
If Konqueror or another web browser is left alone:
And Firefox is *not* the default browser, the box in Preferences to
always check if it's the default is *not* checked, both Firefox and
Thunderbird, so far, have behaved as expected. With the box unchecked,
Firefox obviously does not prompt for it to become the default.
Once Konqueror or another web browser is launched:
Firefox, when launched afterwards, will again prompt to become the
default and once again, Thunderbird will create 'Blogs & News Feeds' and
not open links when selected.
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Linux. A Continual Learning Experience.