Hello,

You are not alone. Firefox has some issues recently and users are loosing all add-ons data, not just in my add-on. See for example this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1530660

But there is way to recover it even if you don't have backups. Firefox in version 66 migrated all add-ons data to a new storage and during this process they created a backup file - "storage.js.migrated".
You can use this file to restore your data, see this article:
https://blog.mozilla.org/addons/2018/08/03/new-backend-for-storage-local-api/
After you located the file, you can actually directly import that file into my speed dial - just rename it to "xxx.group_speed_dial" and then import.

And lastly, maybe you Firefox just switched profiles. You see, Firefox 67 started to use separate profile for each installation - this is a good thing to prevent some serious issues, however for some users this actually caused "data loss" because their Firefox was suddenly using a new profile. But all you need to do is switch to a correct profile. See this article for more info:
https://www.bleepingcomputer.com/news/software/firefox-67-switching-to-empty-profiles-causing-data-loss-fears/

Best regards,
Juraj Mäsiar