Hey, please provide more informations (firefox version, page where this is not working). Will try to replicate and fix if this is a bug.

Edit:
Thanks for the provided informations, I'm checking this now on FF 68.0.2 and all works fine, probably there is a conflict with other installed extensions, could you add issue on the https://github.com/trembacz/xpath-finder/issues ? With that list I will be able to investigate this more deeply.