You are using an out-dated version of Firefox.

Some add-ons are not compatible with older versions of Firefox after the add-on is updated.
I can confirm that it works with the newest version of Firefox.

Please try updating Firefox to the most recent version.

If updating Firefox does not work, you can try reinstalling the update from here.

If reinstalling does not fix the issue, there may be a conflict between add-ons. You will then need to disable all other add-ons to see if Flagfox will then work.

If Flagfox works after disabling all add-ons, you will need to try each add-on one-by-one to see which add-on is causing the issue.

NOTE: Firefox does not create or maintain add-ons. If you would like to contact the add-on developer, please seek further assistance here or email the developers at daveg.extensionfeedback@gmail.com.

Many thanks - I had looked but not found it, probably through my own idiocy. Now installed and working perfectly.

I am glad to hear that your problem has been resolved.

I hope you continue using our products and thank you for contacting Mozilla Support.