Today, Desktop Firefox Sync suddenly claimed "Wrong Recovery Key" and told me to Reset / Update / Unlike device. I unliked - relinked and it appears to work. Several hours later, the same problem happened again. I had several crashes today due to a JS bug in Nightly. I do not know if the crashes are related to this problem, but I would expect the stored config to survive a crash.
Actually, it seems to lose the Sync key at each restart.
I seem to have this setting in user.js, not sure if related or how it got there: user_pref("services.sync.deletePwd", true); I enabled sync-ing of Preferences quite recently. The other Sync-ed PCs are a Beta desktop client and Android Nightly.
This does not happen with Aurora on the same profile. It does not even get the Wrong Recovery Key message. So this is a regression in today's Nightly.
Stored config persists, but an in-memory value doesn't get generated correctly until the first sync. Please try with current m-c or tomorrow's nightly.