SUMMARY

Ok, I used Vivaldi for a while, set it as my default browser - until I got the issue solved with FireFox which made it unusable (13 windows, 1000+tabs, but the real culprit was an add-on that hogged resources even at idle) for a while. I changed my default back to FireFox, but Konsole opens Vivaldi.

STEPS TO REPRODUCE
Not sure - there's things to consider.
1. With Linux Mint 19.3 XFCE (yes, I'm not using KDE - although I tried install it, got advice from official help channel on IRC that I should install simply "plasma-desktop", but when I logged in the newly installed DE - well, it wasn't obviously all there. The functionality was broken, stuff missing and the theming looked like there was none. I have no idea if this has anything to do with it - I still use XFCE).
2. Installed Konsole (before trying to install KDE Plasma I'm fairly sure)
3. At some point I set default applications (in XFCE's control panel) so that instead of firefox it would open new tabs in Vivaldi.
4. Later switched back. Now all applications open links in firefox again - except for Konsole, which starts Vivaldi to open a tab, opening my 4 windows and 200+ tabs of my Vivaldi session, at worst case causing my machine to run out of RAM.
5. I have no idea where Konsole picks what browser to launch... doesn't use exo-open for sure :D

OBSERVED RESULT

Links and documentation opening in browser that I've only temporarily set to as default, Vivaldi, and returned the default browser to FireFox. This selection was done in XFCE DE's settings, which I thought would set some global default that all modern applications would see, but since XFCE is GTK-based and uses it's own config system, maybe KDE/Qt applications DON'T get a clue. Just baffles, why does it open Vivaldi then? Not the usual "default", closed source browser from 3rd party repository.

EXPECTED RESULT

Links would open in browser set default in current DE in use, *and/or* using exo-open to open URL in browser - that seems to always be the same I've set it to in Default Applications of DE settings.

SOFTWARE/OS VERSIONS
Linux Mint 19.3 XFCE
Konsole version: 17.12.3 (it seems this is old not available in selection list above, seems strange for Mint, even though I'm one major release upgrade behind).

ADDITIONAL INFORMATION

Where does Konsole pick information, if there is no Desktop Environment: that's what this makes me ponder as I'm feeling a drift towards going back to plain window managers, and the mandatory libraries and stuff needed to run GTK and Qt applications when Gnome, MATE, XFCE, KDE or other DE's are gone. Need to know where can one change what browser/executable should Konsole launch for URL's?

I think there should be a setting in Konsole, not just somewhere in the DE's settings - I don't even have the DE, in working order anyway for sure.