Clicking magnet: links from Firefox 18.104.22.168 will:
1) If FrostWire is open, do nothing.
2) If FrostWire is not open, it will open FrostWire and then do nothing.
At no point is the linked file added to the queue for downloading. Note that this has been tested using various magnet: links from http://www.magnetmix.com
(When I say "does nothing" I should really say the screen flashes quickly)