There are a number of large repositories in existence.
https://hg.mozilla.org/mozilla-central/ is the mainline of Firefox development. It has over 200,000 changesets and 100,000 files.
https://hg.mozilla.org/releases/mozilla-beta tracks the releases for Firefox Beta. It has 600+ topological heads and 1500+ tags.
https://hg.mozilla.org/mozilla-unified contains several heads of Firefox in chronological order. It is using generaldelta + aggressivemergedeltas.
https://hg.mozilla.org/experimental/mozilla-unified-treemanifests is a conversion of the mozilla-unified repo to treemanifests.