builder.fluidproject.org has been taken down and there are no immediate plans to replace it. Currently all builds are expected to be generated directly from the grunt build scripts.
I looked into this issue a little and determined that this seems to be a known problem with IE8 and animated gifs. Is there another way to include a spinner that wouldn't cause this problem in IE8?
Spinner not spinning
Steps to reproduce:
1) Open the builder page
2) Select some modules
3) Click the download button
Notice that on the download page, the spinner does not spin.