MozillaZine has a page describing how to add new protocol handlers here. On Windows, it looks like it just amounts to a couple minor modifications to the registry. Open Notepad and paste in the following:
@="URL:Custom Telephone Protocol for Qutecom VoIP Softphone"
@="\"C:\\Program Files\\qutecom\\qutecom.exe\" -c \"call\\%1\""
Save the file with a
.reg extension (e.g.
tel-url.reg). You will need to actually type out the
.reg, otherwise Notepad will assume
.txt. You can then import these settings into the PC's registry by double-clicking on the file you just created. I don't know if Firefox reads these registry keys directly, or if it depends on Explorer for that, so it's possible that you may need to restart Windows for the change to take effect.
The process for Mac and Linux is different, and involves setting the following preferences in Firefox itself: