Thanks for flagging this — you were right. The bug was that the
extension released the file data right after the download started
instead of waiting for it to finish, which caused downloads to fail
partway through.

I've already submitted a fix for this (version 1.1) — it's currently
in review with Mozilla and should be live within a day or two. I'd
really appreciate it if you could try the updated version once it's
out and let me know if it works for you now.