I could use a little help from an NSIS expert

DominoWebDAV 0.1 will be up soon. Part of the application is a little helper application, that knows how to deal with the webdav:// protocol. As "good citizens" we want to make the information available, that the helper is installed by amending the user_agent string in the browser. We add something, not alter, so the typical browser sniffing scripts won't break (Note to all who still use them: start sniffing the DOM).
In Internet Explorer the user_agent extension are defined by a registry entry (This is e.g. how IE/IIS "knows" that a .net runtime is installed):
HKLM "Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform" "Some funny string" All entries here end up in the user_agent. In Firefox it is a little trickier: the following steps are needed:

1. Is Firefox installed?
2. Locate the profile directory (per user)
3. Read the profiles.ini and locate the current user profile
4. Look for the file user.js, create it if missing
5. Add the line user_pref("general.useragent.extra.funnySoftware", "Some funny string"); if missing

I managed to get the part with the Registry going, but I could need some help on the Firefox part. Any takers?