Linux has many ways of "marking" an application "as a default". You can check some of them using following commands:

1. xdg-settings get default-web-browser
2. ls -o /etc/alternatives/*browser*
3. ls -o /usr/bin/*browser* (some of these are symbolic links to the previous list item)
4. xdg-mime query default x-scheme-handler/https
5. If you're on KDE grep BrowserApplication ~/.kde/share/config/kdeglobals

I've had the exact same issue as in question How do I change the default browser that konsole opens URLs with?.

I'm running KDE 5.13.5. I did following steps:

1. Uninstall Chromium
2. Make sure all of the four items listed above point to Firefox.
3. Go to "System Settings > Applications > Default Applications > Web Browser"
4. Change setting 'Open http and https URLs' to "in an application based on the contents of the URL"
5. Click an https link in Konsole
6. Install Chromium
7. Click an https link in Konsole

Actual result: after step 5, link is opened in Firefox; after step 7—in Chromium.

Expected result: link is opened in Firefox both in step 5 and in step 7.

So KDE decided, that this particular URL (this one) is better suited (see step 4) for Chromium than for Firefox, despite my efforts to mark Firefox as the default browser in five different ways. If in step 4 I choose "in the following application > Firefox", then everything works as expected, but I want to have it set to "in an application based on the contents of the URL".

Which method of marking a browser as default did I miss? How do I convince KDE that this URL is better suited for what I tried to mark as my default browser?