The Firefox-iOS team recently noticed that this metric generates an error when following the looker link:
https://dictionary.telemetry.mozilla.org/apps/firefox_ios/metrics/firefox_home_page_synced_tab_opened_origin
The Explore that generates the error:
https://mozilla.cloud.looker.com/explore/firefox_ios/metrics?qid=c2hrzuX3AcwnMZOs2fbSZ3&toggle=vis
After speaking to @fbertsch it seems that this is missing the associated dimension metrics__labeled_counter__firefox_home_page_synced_tab_opened_origin missing from the metrics view.