you copy a URL wrapped over multiple lines from somewhere and try to paste it into the address bar, you will end up only with the first line of it. To fix it, go to about:config and change editor.singleLine.pasteNewlines setting to 3 or add:
to your user.js file. Now all the line breaks will be removed upon pasting.
Setting that value to 2 converts the newlines into spaces. Extremly useful.