There’s an extension for Firefox called Twinklefox. However it doesn’t run under FF 3.5. I’ve bumped the version in install.rdf, but twinkle won’t get called correctly none the less.

I browsed FF’s settings and … made a little wrapper script to call twinkle:

$ cat /usr/local/bin/twinkle_callto
# firefox will call twinkle_callto like this:
#   twinkle_callto callto:+41812528006

number=`echo "$1" | sed 's/callto://'`
exec /usr/bin/twinkle --call "$number"

There you go. Very easy.

Go to Preferences->Applications and add this script to handle “callto” URLs. That’s it. You will be able to click on a callto: URL and be handed over to Twinkle.