Firefox35 displays pngs erratically

I have been using firefox3.5 now for some months and noticed that some images, notably in the png format, do not display correctly: the images are not displayed at all or display in part whereby the rest of the image shows a black rectangle. Does anybody else suffer from this problem?
Desktop: xfce 4.2
OS: FreeBSD 7.2

---------- Post updated at 05:53 PM ---------- Previous update was at 09:10 AM ----------

The only workaround that I currently have is reverting back to a previous FF3 version, which is a minor drawback.