Russo 2021 identifies these abrupt stem swellings on fireweed as galls of Mompha "unifasciella," a species complex requiring DNA work. But Mompha sturnipennella also causes a stem gall on this host per https://ukmoths.org.uk/species/mompha-sturnipennella, and iNat has several observations of "Kentish cosmet" in Canada.
”- Gallformers Contributors: (2024) Gallformers ID Notes©
Reference: https://inaturalist.ca/observations/94211706