I did a reboot and before anything else I went to the Properties of the desktop link and now it DOES show FF under the open with tab (when it wasn’t before). And I tried three FF links (without going by opening FF first or going through bookmarks) and the relevant pages did open with FF.
I was all OK 'til some recent update… don’t know why.
The change Pavlos suggested had no impact on Slack and it did solve the sudden html link weird behaviour.
p.s. I am using Slack for teamwork, since the chat tool provide for creating separate channels for each distinct files/topic. We didn’t find any equivalent as of yet.