Open the msi with an editor (I use orca) and go to the "feature" table, change the "Attributes" column to "24" for each set of associations you want by default.
Bumping the thread to ask for MSI testing. I just installed a new component on our buildbot machine to try to fix Bug 8858 , and also enabled MSI building for 2.1 nightly builds. Please test nightly builds as they appear in the next days and report success/failure. Thanks! MSI installer is working ...
Looks like the nightlies are now including an msi installer. I've been banging my head against a wall, trying to get the msi to set the file associations for all file types by default. (Since I'm doing this as a silent install) Also I had to edit out the condition to disable the install until the EU...