In Firefox, if you copy something that has line breaks in it and then try to paste it into a form element which only takes a single line, Firefox ignores everything after the first line break. The time this is most annoying is when you are trying to copy an address (from multiple lines) into an address on a single line (like a mapping site). Happily though, Firefox allows you to change this behavior.

Type about:config in the address bar of your browser and scroll down to the line called editor.singleLine.pasteNewlines. Double click on the entry and you can enter a number from 0 - 4. Here's what the values mean:

0 - Paste content intact including newlines (this is the default in Linux).
1 - Paste the content only up to (but not including) the first newline (the default for non-Linux).
2 - Replace each newline with a space (probably the one you want).
3 - Remove all newlines from content
4 - Substitute commas for newlines