For what it's worth, the only part of the HTML5 specifications that deals with line breaks in title
attributes states:
If the
title
attribute's value containsU+000A LINE FEED (LF)
characters, the content is split into multiple lines. EachU+000A LINE FEED (LF)
character represents a line break.
So you could break up the title
attribute on the server-side, although this is unlikely to be a desirable solution. Alternatively, you can try inserting zero width spaces, though browser support for this may also be inconsistent.
Use escaped n, example:
<input type="text" title="first line \n second line"></input>
No, you cannot make Firefox deviate from its way of rendering title
attribute values; cf. to How can I use a carriage return in a HTML tooltip?
The tooltips created by using title
attributes are of questionable usability (tiny font that cannot be easily increased by the user; disapperance after some seconds; etc.), so they are suitable (at most) for short advisory titles (mainly for links), which not cause a need for wrapping.
For longer tooltips, consider using CSS techniques. Or just put the tip into the text proper.