Only if you have "Bookmarks" option enabled in Firefox Sync, the addon will store the currently viewed website URL in your local bookmarks folder, which then immediately will be synced by Firefox Sync. See this official Mozilla support KB article on how to delete synced bookmarks (disabling bookmarks syncing deletes all bookmarks from the Mozilla servers): https://support.mozilla.org/en-US/kb/sync#w_when-does-my-data-get-deleted