I just found out that Firefox has a preference to have all the newlines stripped when you paste a URL which stretches over multiple lines (via Jeremy Zawodny). This means that if you copy a URL out of an email which has been hard-wrapped to 72 characters (or whatever it is) and spans over multiple lines, everything will just work (instead out just giving you the first line of the URL).

Now, why on earth doesn’t every browser do this by default?

For what it’s worth, go to about:config and set editor.singleLine.pasteNewlines to 3 (1 is the default, 2 apparently puts spaces in place of the newlines).