The following layout renders properly in Chrome, Safari, IE & Opera. In Firefox, the breaks are ignored.
This is the expected result:

My preference is a CSS tweak that will get it working in FF (and keep it working in the others), but I'll take another approach to achieve the same results, if necessary.
JSFiddle
<!DOCTYPE html> <html> <head> <style> ul { list-style-type: none; margin: 0; padding: 0 } ul li { clear: both; } ul li * { float: left; } ul li div:first-child { width: 100px; } </style> </head> <body> <ul> <li><div>One</div><div>A</div></li> <li><div>Two</div><div>B</div></li> <li><div>Three</div><div>C</div></li> <li><div>Four</div><div>D - 1<br>D - 2<br>D - 3<br>D - 4</div></li> </ul> </body> </html>