How can I make Firefox word-wrap text/plain files?

FF 31. Open a new tab.

Enter about:config in the Awesome Bar.

Search for view_source.wrap_long_lines and set value to true.

The Toggle Word Wrap Firefox extension might work for you.

If you just need to wrap plaintext files, try Ctrl-U (then View -> Wrap Long Lines, if not already checked). This solution is cleaner than an add-in with plaintext files, but sloppier with text embedded in html.