So Mozilla are fucking retards and removed any convenient way to add a custom URI handler.
- You can't add one by just following a, say, ssh://host link. Firefox doesn't ask you what to use to open the link anymore, just goes HURR DURR UNKNOWN PROTOCOL.
- You can't add one in Preferences -> Applications. No “add” button anymore.
- You can't force it through about:config like you used to.
Here is how you can do it though. Find a file called mimeTypes.rdf in your firefox profile and add the custom protocol.
First, add the scheme in the 'urn:schemes:root' block:
<RDF:Seq RDF:about="urn:schemes:root">
...
<RDF:li RDF:resource="urn:scheme:telnet"/>
...
</RDF:Seq>
Then, add a new block:
<RDF:Description RDF:about="urn:scheme:telnet"
NC:value="telnet">
<NC:handlerProp RDF:resource="urn:scheme:handler:telnet"/>
</RDF:Description>
Now restart the browser and go to Preferences -> Applications. telnet will be there.
And to you, the person who decided to remove this function: Choke on your own cock.