Hello,
Sadly, storage breakage is a Firefox issue. Addons these days can't access drive or database directly, they only instruct browser to store or load data. If thumbnails are missing, it's likely result of a broken IndexedDB database.
More info:
https://github.com/fastaddons/GroupSpeedDial/wiki/Fixing-broken-dials-page

Regarding the Pro version, well it's to cover the development cost. Since I'm not payed by Mozilla, I have to earn money somehow, and most of my addons are free, so this one is the only one that actually keeps me and my work alive :).
It's a pretty bad paying job being an addon developer, that's for sure :(.

Best Regards,
Juraj Mäsiar