Google (sometimes) doesn't work in Firefox

Sometimes when I want to search something on Google in Firefox, it doesn't work. When I hit the search button, the page loads but then a blank page appears with a smaller Google logo and the buttons in the top left disappear:
Attachment 3322

A few things I've noticed:

• I'm feeling lucky works
• Going back and going to image results and then back to web works
• Only happens in Firefox (I think since 3.5 until 3.6.3)

Has anyone else seen this? Is it a bug? Thanks!