Linux Mint 17.4, 64 bit, KDE Plasma 4.x, firefox 59.0.2

I am using since long the bookmark widget. Suddenly the bookmarks stopped working without a change in the system.

Observation: When I click a bookmark, firefox is indicated as starting up but it does not. Even with firefox running, I observer the same behavior.

Note: LInks in an email, LibO file etc work, only the Bookmark widget the links show above behavior.

I appreciate any hint to solve the issue and get back to normal function.