This script replaces partial transparency in all layers of an image with the chosen color. You can download the script here: FP's semi-flatten all layers script
Gifs are one of the most common forms of animations for the web. However, they are limited to 256 colors and "all-or-nothing" transparency. This limitation in transparency sometimes creates hard or "jagged" edges when placed on background. Within GIMP is a tool called semi-flatten which allows the user to select a color similar or equal to the background it will be used on and will blend the hard transparent edges with the color so it appears anti-aliased. The preview picture shows an animation with the left side being left untouched (notice the tops of the flames are hard edged). The right side of the preview has been semi-flattened to match dA's background color and appears much smooter.
The script works only on RGB animations with an alpha channel. Such as when you first create the animation. It doesn't really well with previously created animations that were not semi-flattened. This is not a flaw of the script. It's just the way it is.
Download the zip file, open the zip file by double-clicking on it and copy/paste the script to your GIMP scripts folder (ie...C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts). Refresh your scripts and you're ready to go. The script can be located under the Layer > Transparency > Semi-Flatten All Layers...
Let me know if you have any trouble or need additional information.