Kubuntu 18.04 can't set default web browser

Links in Telegram have been opening random apps (image viewer, Calibre, etc), so I checked System Settings > Applications > Default Applications > Web Browser.

Open URLs "in an application based on the contents of the URL" sounds like the culprit, so I changed to "in the following application (Firefox)".

However after leaving this screen or rebooting (both) the setting reverts:

I ran system settings from console, but I couldn't see any obvious errors.

Update: I ran a fresh install of Kubuntu 18.04 in a virtual machine and tried this, with the same result. Did all available updates and rebooted, tried it again - same result - even though it was on a fresh, stock system.

Solution 1:

Not ideal but I used the with the following command: option on Default Applications dialog in Kubuntu 18.04. For Firefox I would assume this would work: /usr/bin/firefox %s. I prefer Google Chrome so I used: /usr/bin/google-chrome %s

Solution 2:

What was helping me: (I am using Kubuntu 18.04 LTS) find the following file /home/USERNAME/.config/mimeapps.list and edit it like this:

[Default Applications]
application/x-desktop=doublecmd.desktop;
inode/directory=doublecmd.desktop;
inode/symlink=pcmanfm.desktop
text/html=google-chrome.desktop
x-scheme-handler/http=google-chrome.desktop
x-scheme-handler/https=google-chrome.desktop

The last two x-scheme-handler/... lines were empty for me, so I wrote google chrome there as I prefer chrome.