My version of Firefox is 3.5.9, but I've had this issue since single-image tabs started showing a thumbnail of the contained image.
Typically, a tab containing a single image will use a thumbnail of the image as the icon before the tab title. Sometimes, though, this icon will be the generic “shaded sphere on blank page” (or perhaps whatever your theme defaults to). I'm guessing this happens whenever height or width is larger than 1024, but I seem to recall (perhaps erroneously) exceptions to this rule. Anyway, hopefully someone will know the definite answer to:
Why does this happen?
Images showing thumbnails
Images not showing thumbnails
I would appreciate even more a link to the source file or even a patch that explains this behaviour, if it's not a bug. I've tried to locate such a code snippet, but I was unsuccessful.