Yes, unfortunately Mozilla does not provide a way to handle semantic versioning, so an update from 0.x to 1.x will be automatically applied without any question.

You can try to install the old version from https://addons.mozilla.org/en-US/firefox/addon/rested/versions/ and do an export if your collections are still there.

The clearing of the store is unfortunately unavoidable, as the 1.0 update both upgraded RESTED from a firefox extension to a webextension, and changed the storage backend from IndexedDB to chrome.storage. No migration other than manual user migration is possible.

Sorry for your pain