The error was occurring in this line, something to do with the Firefox FileReader internal implementation.
So, I changed how the zip file is built.
I tested it on latest Chrome, Firefox and IE 11.
- Imported a resource pack (OCD)
- Refresh the page to check if the pack is still there
- click zip and checked some of the image file
It worked for me on my computer.
Let me know if it works now for you, you may need to clear your browser cache (ctrl+shift_del on windows) to force the browser to get a fresh version of the site scripts.