I wish I could say you are doing something wrong, but unfortunately FF, especially the 3.5 series, has had some problems. Many others have reported numerous issues with FF in the last 6 - 8 months.
From an outsider's point of view, I fear that FF is becoming too Windows focused. There seems to be less and less being done with the Linux version.
It took an unusually long time for Iceweasel 3.5 to get into sid. This leads me to believe that there was quite a bit that had to be done to FF 3.5 to get it up to Debian's admittedly high standards.
Insanity: Doing the same thing over and over and each time expecting a different result.
Democracy is 2 wolves and a lamb voting on the menu. Liberty is an armed lamb protesting the electoral outcome. A Republic negates the need for an armed protest.