Since upgrading to 3.6.8, I have noticed that Firefox crawls to a virtual halt immediately after clicking on links that either open or download PDF files. It doesn't freeze but just runs really really slow e.g. it will 5-10 seconds to recognise a menu click. The only way of getting out of it is to force-quit Firefox and re-open the program. All other programs running at the same time seem to run OK so it is something specific to Firefox not a general System problem. There were no such problems with earlier versions of Firefox but I did notice that all Firefox versions I have used tend to slow down the longer it is used regardless of how many windows or tabs are open. I would usually just quit and re-start after a few hours . . . but the problem mentioned above with PDF's and 3.6.8 is different.