I'm having the same trouble. Opened a valid png file, selected and copied a portion of it's text, pasted that into new layers with transparent backgrounds, made my alterations to each new layer, saved successfully to both aPNG and aGIF file, renamed to PNG and GIF respectively, and both "play" in Firefox except:
the PNG animates but the original "background" pic I started with get's covered over by the subsequent frames. Unfortunately, everything in those frames was supposed to be transparent, but are a dark grey... which is the default workspace color of my theme in FF. The idea here was to have a static background (the original png) with text that gradiated from one end to the other.
the GIF animates but again, same troubles as above except the added "transparent" frames cover over the original background pic in white.
I've tried several methods to find a work-around, to no avail. (about 2 hours worth now!) I duplicated many layers of the original PNG image and only altered the text in each frame (layer) and it still does the same. I even cleared FF's cache and also renamed the faulty file just to be sure the browser wasn't caching and old copy. ;(
