It can be done, but not via the MicroB UI. You need to put following lines into your user.js file (it's not created by default, so create it in ~/.mozilla/microb/):

user_pref("capability.principal.codebase.p0.grante d", "UniversalXPConnect");
user_pref("capability.principal.codebase.p0.id", "file:///home/user/some_dir/your_file.html");
user_pref("capability.principal.codebase.p0.subjec tName", "");

This tells MicroB that the user has already given the UniversalXPConnect privilege for the file your_file.html to be able to read and write local files.

See http://my.opera.com/Andrew%20Gregory...how.dml/405642

Hope this helps