Do profile migration.
When this method is called, a default profile has been created; XPCOM has been initialized such that compreg.dat is in the profile; the directory service does *not* return a key for NS_APP_USER_PROFILE_50_DIR or any of the keys depending on an active profile. To figure out the directory of the "current" profile, use aStartup.directory.
If your migrator needs to access services that use the profile (to set profile prefs or bookmarks, for example), use aStartup.doStartup.
The startup code ignores COM exceptions thrown from this method.