Disable "open with" option on Firefox downloads (force Firefox to always save all files)

Solution 1:

Making a master mimeTypes.rdf file that you deploy to users is the best solution

Open mimeTypes.rdf in an editor that handles XML style docs well (such as Notepad++)

Find and replace first line with second:

NC:saveToDisk="false"
NC:saveToDisk="true"
  and
NC:alwaysAsk="true"
NC:alwaysAsk="false"  

I would also "seed" your master mimeTypes.rdf with some of the exotics your users have accumulated:

1. Copy mimeTypes.rdf from the 2-3 users with the heaviest usage of webmail attachments
2. Compare users' types to your master:

cmd.exe

find "<RDF:li RDF:resource=" Master_mimeTypes.rdf > MasterMimes.txt   #generate list of mimetypes
find "<RDF:li RDF:resource=" User1_mimeTypes.rdf > User1Mimes.txt     #generate list of mimetypes
fc MasterMimes.txt User1Mimes.txt                                     #compare lists of mimetypes

If you find any new types, grab the whole stanza from the user's file and add to master.

Solution 2:

In about:config there is now an option browser.download.forbid_open_with. Just set it to true