Labs/Weave/Developer/HowToSynchronizeAddOnPrefs

Weave Sync by default syncs the preferences that are accessible from the Firefox UI. Add-ons can leverage preference sync to have user-set add-on preferences synced so that changes to add-on settings will propagate to other synced clients.

A preference will be synced if a corresponding prefixed-preference is set to true.

The prefix (note the trailing period) is extensions.weave.prefs.sync.

So if extensions.weave.prefs.sync.browser.startup.page is true, the browser.startup.page preference will be synced.

An easy way to set these preferences to true is to add the entry to the default preferences for the add-on. You can read about that at https://developer.mozilla.org/en/Default_Preferences