On current Debian Wheezy/Testing, icedove does not ask anymore which http(s) handler it should invoke, but rather uses x-www-browser as environment value (which is of course iceweasel, and i do want to use chromium, but only in icedove/thunderbird for now).
So, how to set that from the settings? Well, no idea and google only points to the config editor itsself.
So let’s do it that way.
Look for network.protocol-handler and double-click the warn-external.{http|https|ftp} handlers in order to modify them to “true”.
Next time you click on an url, you will be asked again which application to invoke. Click “Choose”.
Now this is really ugly as this does not show a fancy menu, but requires you to look for the application binary yourself. Click the left icon to invoke an address bar and type /usr/bin/chromium to point to Chromium’s binary.
Once selected, also mark this as remembered choice and click ok.
Voilà.