Good find, hadn’t thought of the OS being the issue given that Firefox was playing ball but that makes sense. Thank you! This give me a push in the right direction to where I found folks talking about problems accessing fonts when Librewolf was installed with dnf on Fedora.

Following the same strain of thought I decided to try installing Librewolf from Flathub; curiously it accesses and displays the fonts correctly therefore solving my issue. My original method of install was via the debian based systems repo.