Bug 1757375 just added a thread attribute on all redux source objects.
This should help get rid of various selectors which were computing this data out of the source actors related to a the given source.
We can replace these couple of selectors with a simple attribute access!

https://searchfox.org/mozilla-central/rev/684aefab97202faa982dd898f1d99208a46f12b0/devtools/client/debugger/src/selectors/threads.js#51-62

export function getThreadForSource(state, sourceId) {
  const actors = getSourceActorsForSource(
    state,
    isOriginalId(sourceId) ? originalToGeneratedId(sourceId) : sourceId
  );

  if (!actors || !actors.length) {
    console.error(`Error no source actors exist for source ${sourceId}`);
    return null;
  }
  return getThread(state, actors[0].thread);
}

https://searchfox.org/mozilla-central/rev/684aefab97202faa982dd898f1d99208a46f12b0/devtools/client/debugger/src/selectors/source-actors.js#59-77

const queryThreadsBySourceObject = makeReduceAllQuery(
  actor => ({ thread: actor.thread, source: actor.source }),
  actors =>
    actors.reduce((acc, { source, thread }) => {
      let sourceThreads = acc[source];
      if (!sourceThreads) {
        sourceThreads = [];
        acc[source] = sourceThreads;
      }

      sourceThreads.push(thread);
      return acc;
    }, {})
);

// Used by threads selectors
export function getAllThreadsBySource(state) {
  return queryThreadsBySourceObject(state.sourceActors);
}

Surprisingly, this patch doesn't trigger any improvement in DAMP:
https://treeherder.mozilla.org/perfherder/comparesubtest?originalProject=try&newProject=try&newRevision=876b11e2b43580b9c2132a007e1617330ec1b10a&originalSignature=3130994&newSignature=3130994&framework=12&originalRevision=929c91f244b5c939c2f0b99357ead3cd243e88d6&page=1&showOnlyConfident=1

https://hg.mozilla.org/mozilla-central/rev/10e5ac539c0d