FirstStartup provides an optional mechanism to run synchronous services
before the UI appears. This is intended to be called as a post-install
action from an installer, such as the Windows Stub Installer, to allow
for downloading critical data and performance tuning before the first
run of a new Firefox install.
Differential Revision: https://phabricator.services.mozilla.com/D43364
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extra : moz-landing-system : lando