I was able to use personas in LO6.0.7 on LinuxMint 19.1 Cinnamon. Due to a bug in LO (drops music in the slides of Impress), I removed LO and reinstalled a more recent version from the FlatPak-repositories. LO is working fine and the problem in Impress has been solved. However, installing personas doesn’t work at all. Entering the name of the persona, nor entering the according URL doesn’t work.
I found a folder with older personas in $HOME/.config/libreoffice/4/user/gallery/personas
I tried to edit the persona-keys Tools > Options > Advanced > Open Expert Configuration and set the Persona-key to own
and PersonaSettings to the location of a persona-image, but that didn’t work either. Any idea how to install the firefox-personas?