default=<selector>
See [https://firefox-source-docs.mozilla.org/tools/try/selectors/index.html this documentation] for more information on all the available selectors, and [https://firefox-source-docs.mozilla.org/tools/try/presets.html this document] for a list of available presets (derived from [https://searchfox.org/mozilla-central/source/tools/tryselect/try_presets.yml try_presets.yml]).