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 compatibility concerns as well.
For config (assuming you mean prefs in about:config), you can add your own prefs to be synced with about:config. If you add a new boolean pref with a value of true using the pattern `services.sync.prefs.sync.<pref you want to sync here>` it will be synced. e.g. to sync `browser.foo.bar` you'd create `services.sync.prefs.sync.browser.foo.bar`.
Disclaimer: I work on Firefox sync.
[0]: In the future this may be more complex, and you may need to flip an additional pref to get this behavior for non-default prefs.