If the first page loaded is a file which could be handled by an installed plugin, it's not displayed because the MIME types database hasn't been updated yet with the types supported by the installed plugins. To test this, open the test launcher with a link to a non-HTML file that should be handled by a plugin.
Talked with marcoil, this bugs can't be reproduced with r34688. May be another change fixed this. Closing as WORKSFORME.