In the Firefox downloads dialog, the containing folder is opened in Nautilus, which is a nuisance as well as an eyesore on my nice KDE4 desktop.
Two solutions are proposed on the web.
The first (from Rubylution), more elegant, is to fiddle with the about:config in Firefox by defining 3 new preferences: network.protocol-handler.expose.file = true (Boolean), network.protocol-handler.external.file = true (Boolean) and network.protocol-handler.app.file = konqueror (String). See the link for detailed instructions. Sadly, this had no effect, I still got Nautilus.
By the way, there is no way to delete erroneous preferences from about:config in Firefox, you have to edit ~/.mozilla/firefox/<yourprofile>/prefs.js by hand and restart Firefox.
The second (from hombrepac.com.ar) method stealthily substitutes Nautilus with Konqueror in the KDE mimetype handling. This basically makes Nautilus unusable, which might be a problem if you are an occasional Gnome user.
Well, I'm not - and this time, it worked.
Update: Newer Firefox versions (confirmed with FF 3.6.6) apparently detect KDE's default file manager - anyway, the downloads folder is opened in Konqueror without me having to do anything about it.