Firefox sync about:config

It synchronizes some settings – those that are enabled in services.sync.prefs.setting.

The user preferences are stored in prefs.js in your Firefox profile.

Firefox synchronizes about:config settings for which there exists a true boolean entry called services.sync.prefs.X, where X is the setting name.

So if you wish to synchronize a new setting X, simply make a new boolean entry called services.sync.prefs.X and set it to true.

In fact, the correct boolean entry to enable synchronization for entry X would be services.sync.prefs.sync.X; there are two .sync's.

See https://developer.mozilla.org/en-US/docs/Archive/Mozilla/Firefox_Sync/Syncing_custom_preferences.