I use Ubuntu 18.04 with KDE. My default PDF viewer is Okular and it has worked as intended (when I open PDF, it laucnhes Okular).
That has changed recently: If I now open PDF i.e. from Dolphin, Okular app opens just normally. However, the default app for Firefox and Thunderbind seem to have suddenly changed to Evince. No configuration has been touched.
Any idea why FF and TB now use different app as default? I tried even removing Evince, but then FF and TB opened PDF with Inkscape.
My own suspect is that this is some new bug that come with some recent upgrade.
Any ideas? Thanks!