But I just noticed you said linux, this was a feature Mozilla added at our request for mac & windows:
https://bugzilla.mozilla.org/show_bug.cgi?id=434135 Bug 29250 - offline: mailto handler
For others a trick that to get it to work after that (if it's not right away) is to restart the prism web-client, then a javascript trigger (when prism is running) & the url trigger (to launch prism when it's not) seem to take better effect after - we're not sure where the bug is in that yet, but there is a note about it filed in bugzilla.