firefox should not use mailcap defaults with needsterminal

right now firefox will propose to open all text/* documents,
i.e. "text/x-diff" as listed in /etc/mime.types for patches with "less"
for one /etc/mailcap has "needsterminal" so "less" should never be listed as the default,
using it results in nothing, since less would have to be opened in a terminal.

Expected Result:
 do not provide any terminal based app as default choice, do not even list them.
 instead have a reasonable default, i.e. gedit for text/* or use xdg-open