Trigger Scripts and Install Scripts
Trigger scripts are simple installations that can be initiated from event handlers and other JavaScript code on a web page.
Triggers use the InstallTrigger object methods to compare software versions, install chrome, and perform simple software installations.
Install scripts use the Install, File, InstallVersion and platform-specific installation object methods to initialize, queue, manage, and perform the installation of one or more software packages.
These install scripts are typically located at the top level of the XPI archives in which the installations are stored. A trigger script may trigger the downloading of a XPI, which in turn will use its own install.js script to manage the complete installation.