How do I set downloads to automatically save instead of prompting me to open them?

Question

I recently reset Firefox. Before whenever I downloaded a file, it automatically prompted me if I wanted to open the file or save it to my computer, with saving it to my computer preselected. This was fine because I almost never want to open these downloads automatically, and I like all my downloads to go into my downloads folder on a separate drive with much more space (not the temp folder on C:\). The option "do this automatically for files like this from now on" was useful for the rare file type that I would want to open automatically (usually because it's a very small music file or something).
Now every time I download virtually any file type, the prompt comes up with 'open the file' preselected, and "do this automatically for files like this from now on" greyed out. As a result, on more than one occasion, I have mistakenly clicked ok and opened huge files automatically after downloading (which has resulted in them popping open even hours later and playing automatically once the download finished). I'd like to be able to select "save it to my computer" and have it do this automatically, but that option is greyed out.
The help available seems to suggest going into options>Applications and changing the action for the appropriate content type, but:
(a) not all the content types I download are listed, i.e. .mov, .mpeg, .rar, etc.
(b) changing the action for the content types I do have (.wmv and compressed/zipped folder only) has done absolutely nothing.
I'm still prompted for each and every file I download, still preselected to open the file, with the "do this automatically for files like this from now on" still greyed out.
I'd like to change it to the way I had before, or at the very least automatically set all downloads to save into my downloads folder.

Answer 1

You can automatically save downloads without seeing the Open/Save/Cancel dialog in the Options:
orange Firefox button (or Tools menu) > Options > General > "Saves files to"
However, I can't think of a way to have the Open/Save/Cancel dialog always default to Save. I think mine suggests whatever I did last for the same type of content, but I haven't paid close attention.
I'm out of time to address the problem with "do this automatically" but there is one suggestion for why it could be grayed out in this article: [[Change what Firefox does when you click on or download a file]].
Is it possible you had a download-related extension before? To compare settings and extensions, please retain the Old Firefox Data folder on your desktop. Aside from checking the extensions in there, it would be interesting to run a text file comparison on your currently active prefs.js file and your old one in that folder.