The squares are some character that is not being displayed correctly - probably quote marks.
From - http://forums.mozillazine.org/viewtopic.php?f=9&t=1195115 : "Squares in firefox's title bar V3.0.8"
Try this-- Click on Tools --> Options --> Content. Click on the second Advanced button. Make certain "Allow pages to choose their own fonts..." is checked.
Freezes and slow performance if only or especially when viewing content from YouTube could be due to download throttling, a poor or slow connection, too little memory, an underpowered graphics card, faulty video drivers, or something else running on your system soaking up the cpu. There are general PC forums that will help you run diagnostic tests to narrow it down.