Probably the dev needs to use a different method to save (effectively download), much as Adobe Photoshop does when it exports files on Firefox. Not hard to do at all. Likely the same for reading files, if this tool needs that.

OPFS provides high-speed read and write for temporary files and non-exported files; again this is what Photoshop uses. There is a 10GB limit per domain currently. I'm not sure this particular app actually needs that, though