I’m having a rather difficult time believing I’m the first one to encounter this, but some pretty deep Googling and searching of the Bugzilla database isn’t turning up any prior mention of it.
When using a transparent GIF as a background image in an absolutely–positioned div, Mozilla does some pretty radical colour shifting. I’ve had this happen on more than one occasion, but I only looked into it a little further today.
update (4/14): — it’s been brought to my attention that this bug has been fixed by Mozilla v1.4. Great news for future releases, but Netscape 7 is still buggered unfortunately.