Using the advice here I was able to work out the following:
1) open a tab and type about:config in the address bar.
2) type ContentHandlers in the filter box.
3) Choose one of the feeds you don't use (Yahoo, for instance).
4) Replace the Title with "Livejournal"
5) Replace the uri with http://www.livejournal.com/syn?url=%s
6) Shut down Firefox and restart.
You can now click on the orange icon, choose " subscribe as live feed" and then select Livejournal from the dropdown list. It'll take you straight to the livejournal syndication page and you can then add the feed to your friends list (choosing a name for the feed if one doesn't already exist).