Hello,
This is a browser "security feature". But Firefox should only display it once.
Note that disabling and re-enabling addon will reset all warnings. Maybe you have some special software that can disable/enable installed addons?
If nothing helps, Firefox Reset likely will - but this will also delete dials data, so you'll need to make a backup first. More info here:
https://github.com/fastaddons/GroupSpeedDial/wiki/Fixing-broken-dials-page#what-if-reinstalling-didnt-helped
Best Regards,
Juraj Mäsiar