For most file types whose download behavior is set to "always ask" the "save as" option is selected by default in the download window. However, for a few others the "open with" option is selected instead. Note that Firefox is not automatically opening some files instead of saving them. I still get to choose between saving or opening them, but certain file types start with "open" selected whereas for others "save" is the default.

What I'm looking for is a way to change all file types to default to "save as" in the download window.