We have to perform the same tests with 1509 user profiles containing bookmarks, history, saved form data, saved passwords, feeds, '''extensions and themes''' compatible with 200x, and extensions and themes '''not compatible with 200x'''. Think of a typical 150x user profile. After the major update the user profile should be kept, and during the update, the extensions and themes should be checked for compatibility.
'''Reference'''
As a reference we should take a look at the test cases already in Litmus. Follow the link bellow, and enter your [http://wiki.mozilla.org/Firefox:1.5.0.x:Major_Update_1.5.0.x_to_2.0.0.x:Test_Results '''test results here''']