The handling of mimetypes is handled by a file called mimeTypes.rdf, with local settings in .mozilla/firefox/profilefolder/mimeTypes.rdf and global settings in /etc/mozilla/firefox/profile/mimeTypes.rdf.
By default the global settings file is effectively empty. Lets replace it by the following:
<?xml version="1.0"?>
<RDF:RDF xmlns:NC="http://home.netscape.com/NC-rdf#"
xmlns:RDF="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<RDF:Description RDF:about="urn:mimetypes">
<NC:MIME-types RDF:resource="urn:mimetypes:root"/>
</RDF:Description>
<RDF:Description RDF:about="urn:mimetype:externalApplication:application/x-zope-edit"
NC:path="/usr/local/bin/zopeedit"
NC:prettyName="zopeedit" />
<RDF:Description RDF:about="urn:root"
NC:de_defaultHandlersVersion="3" />
<RDF:Description RDF:about="urn:mimetype:application/x-zope-edit"
NC:value="application/x-zope-edit"
NC:editable="true"
NC:description="Zope External Content">
<NC:handlerProp RDF:resource="urn:mimetype:handler:application/x-zope-edit"/>
</RDF:Description>
<RDF:Description RDF:about="urn:mimetype:handler:application/x-zope-edit"
NC:alwaysAsk="false"
NC:saveToDisk="false"
NC:useSystemDefault="false"
NC:handleInternal="false">
<NC:externalApplication RDF:resource="urn:mimetype:externalApplication:application/x-zope-edit"/>
</RDF:Description>
<RDF:Seq RDF:about="urn:mimetypes:root">
<RDF:li RDF:resource="urn:mimetype:application/x-zope-edit"/>
</RDF:Seq>
<RDF:Description RDF:about="urn:schemes">
<NC:Protocol-Schemes RDF:resource="urn:schemes:root"/>
</RDF:Description>
</RDF:RDF>The indenting didn't come out nicely. Have a look at at the file itself.