My guess would be that this is a conflict with another addon, or more likely a theme. I would revert to the default Firefox theme, and if that does not fix it, try disabling any recently installed addons.

Edit: Actually first check that you don't have a Firefox Persona installed. I lump them with themes, but I suppose they are technically different. Anyway, they can cause incompatibilities with many addons (like Zotero).

Wow it was a quick fix !! Thank you very much !!! The themes were the offending installation that is conflicting with Zotero fonts.