Earlier quoted context omitted.
The storage is more limited for synced extension data than for unsynced data. IMO this is quite unfortunate, and is mostly a consequence of the implementation of extension storage sync being done separately from the rest of sync (which in retrospect was a mistake, but at the time that wasn't clear). Also, the limits are copy-pasted directly from the limits chrome places on sync, so it's possible there are compatibili…
Oh that’s pretty cool. I’ll try that. I just assumed that this behavior would apply to everything once I turned sync on, and then was disappointed when it didn’t work. I had no idea it was an optional feature. I guess it doesn’t fit the key-value paradigm, but this feature would be a lot more discoverable if there was a “sync?” Attribute next to each about:config item.
A checkbox like you describe was actually discussed in the past, but at this point it’s unlikely. The current design of letting any synced machine change any pref on any remote machine (effectively) has dubious security implications, and has gained an additional hoop you must jump through in nightly.