We do this for some of the components that are shared between Servo and Firefox. Firefox is upstream, and on the Servo side we have automated and manual syncing. The automated syncing mirrors the upstream `main` branch to our `upstream` without changes daily. The manual syncing rebases our changes on top a new upstream version through a manual rebase process. This happens monthly and each sync is pushed to a new branch to maintain history.

Between monthly syncs we push our own changes to our latest monthly branch (which also get manually sent upstream when we get a chance).