associating files with same MIME but different file extensions to differentt applications

I strove to make Firefox ⟷ maff association only for one user (me). So I didn't touch files under /etc.

I did the following:

1. Restore correct association between ZIP-files and archive manager (I did it within default file manager Nemo).

2. Create definition of MIME type. In my case I created firefox-maff-my.xml file in ~/.local/share/mime/packages directory with the content provided below:

   <?xml version="1.0" encoding="UTF-8"?>
   <mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
     <mime-type type="application/maff">
            <comment>Mozilla Archive Format</comment>
            <glob pattern="*.maff"/>
     </mime-type>
   </mime-info>
   

3. Update MIME database

   update-mime-database ~/.local/share/mime/
   

4. Edit ~/.local/share/applications/mimeapps.list. Add

   application/maff=firefox.desktop
   

   line right under [Default Applications].

5. Log out, then log in.

In case of doubts you can make backup of each involved file before editing it. If something will go wrong you can revert everything you did (just restore original files, remove firefox-maff-my.xml and run update-mime-database ~/.local/share/mime/ again).

_{I've faced similar issue in Linux Mint 17.1 Cinnamon. Perhaps this also will work for GNOME (because both follow freedesktop.org).}

As of 2016/Feb/20, this is the correct solution to associate MAFF files to Firefox (or any other files), without affecting the Compressed archive/ZIP association:

# run as sudo

echo '<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
<mime-type type="application/x-maff">
<comment>Maff File</comment>
<glob pattern="*.maff"/>
</mime-type>
</mime-info>' > /usr/share/mime/packages/maff.xml

update-mime-database /usr/share/mime

# now kill X or logout.

The assogiate solution is not valid anymore, as the package has been removed from the Ubuntu repositories after v14.04.

Simple way to do this is by using mimeopen command to open .maff file format using Firefox

Open terminal and type

mimeopen some_file_name.maff

You will be provided with available packages to open the .maff file

Please choose a default application for files of type application/zip

    1) Archive Manager  (file-roller)
    2) Archive Mounter  (mount-archive)
    3) Other...

select 3 from the option and enter the command as firefox

use application #3
use command: firefox

This will open up .maff file in Firefox and will be set as default program to open .maff file