User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

Currently the synchronization does not save the settings of the extensions.

Actual results:

Firefox does not save custom settings for extensions.

Expected results:

I suggest that, if possible, the Firefox sync account should also save custom extension settings.

Thanks for the report, but this is the responsibility of the addons - the chrome.storage.sync API allows addons to sync arbitrary things and they should use this to sync whatever settings make sense for their specific addon.

Please consider opening a bug for the addons which don't sync the options they think you should.

This for me is a problem that should at least be taken into consideration. Do not close the suggestion to improve the sync of Firerox. Firefox in addition to synchronizing the extensions should also synchronize the options/customization's of the extensions made by the user. This I think may be possible since the custom settings are saved in the browser's directory.
I don't know the theme of the API restrictions for syncing, but they should be improved and if you can't by the API change it.
Thank you very much.
Best regards.

Not all preference will make sense to sync. Web extensions can store their settings in one of 2 places - one place syncs, the other does not. So it's entirely up to the addon.