The change from Weave to Sync happened late in the fall of 2010, and all older Weave accounts automatically got a Sync Key as part of the upgrade to the new weave / sync version number. ''Previously it was just called a "key".'' Weave / Sync is intended for synchronizing various sections of your Firefox personal data between different devices, not as a backup service. That said, when you reload Windows and Firefox your need to treat the new Firefox installation as a '''"new device"''', and for that you need the '''Sync Key'''. Without the Sync Key, your data is useless. The Sync Key is used to decrypt the data stored on the Sync server, so no one except for the person with the correct Sync Key can see and use your data.