You can add virtually any preference to Firefox Sync. Not all are included by default, since that would be a massive amount of data to sync.

To add a preference to the Firefox Sync service, you need to create a new boolean preference that is services.sync.prefs.sync.PREFERENCE and set that to true. This needs to be done on all systems that you want to sync this setting with, including systems that you add to Firefox Sync in the future.

For example, in this case you want to add a preference called services.sync.prefs.sync.browser.tabs.closeWindowWithLastTab and set that to true.

To add a new preference, right click one of the preferences. Then select New and then Boolean from the context menu.

Hope this helps.

NOTE: I've found that when you add a preference to the Firefox Sync setting it often resets the original setting and you need to change the setting back again.

Glen.Ford said

I set "browser.tabs.closeWindowWithLastTab" to false on one PC to prevent exit when the last tab is closed. It remains true on the other PC even after syncing. I have to manually change that setting on the other PC. It appears that config data is not included in sync.

Thanks. I consider the question answered.

It makes no sense to sync all of the about:config settings.

However, it would be nice if the non-default settings could all be synced to avoid what could eventually be a laborious process.