I've only just found this thread - I was looking for something else. I have the answer. It's a combination of later versions of firefox, an nvidia graphics card and the opensource 'nv' nvidia driver. Install the proprietary nvidia driver and the problem goes away.
I'm surprised you didn't say it makes posting in a forum such as this - if you move the cursor back and correct typos that is - a complete nightmare.
Seeing as you have only made one post, and four months have now gone by, I guess you may have given up and gone away. I hope not.