You should contact its support.

Thank you, FredMcD. I have found the Flagfox forum < https://flagfox.net/ >, registered, and posted my issue there.

When I learn a solution that works, I will post that information here.

Juanaquena

Good luck.

This is the reply I received to my post about his on the Flagfox Forum < https://flagfox.net/ > (free registration and login required to post):

by DaveG » Thu Sep 28, 2017 3:15 pm Please try installing the latest Flagfox update, version 5.1.29. https://addons.mozilla.org/addon/flagfox/

Also, try clearing your cache.

If that is still producing the error, then please check the Firefox error console (press ctrl+shift+J) for anything that might be Flagfox related and paste it here so I can attempt to see what's breaking.

Firefox handles the actual updates, and seems to be kung fu death grip holding onto caching one of the old version's files past updates and restarts, for some stupid reason. I made an attempt to work around this bug in 5.1.29, and if it's still doing it for some people (it fixed it for a test profile I had), then I'm going to need more info to figure out what to do, especially because it happens so erratically. +++

Also, what I did prior to seeing the reply:

Thu Sep 28, 2017 3:41 pm Earlier today I removed Flagfox as an addon to Firefox. Then installed the latest version. Also, because of other issues, I installed a new version of Adobe Flashplayer. Flagfox seems to be working properly now.

That was very good work. Well done.