Hello,
The error box should contain information about the error - sending it to me could be helpful, more info:
https://github.com/fastaddons/GroupSpeedDial/wiki/How-to-report-issues-(contact-support)

Stopping Firefox updates is not a good idea from the security point of view, especially long term.
Although the error appeared on two devices, there is a good chance the error will be gone after restarting Firefox. And if that doesn't help, then reinstalling my speed dial should 99% help. More info here:
https://github.com/fastaddons/GroupSpeedDial/wiki/Fixing-broken-dials-page
But before you start, make sure to backup your dials data first (it's mentioned in the article above).

If you need further help, contact me here: admin@fastaddons.com

Best Regards,
Juraj Mäsiar