I think I found the problem: my proxy changes the file size in transit. Yours might do the same.
How to check: go to your extension link with and without proxy. Before clicking download, open devtools and see the difference. I open this page Releases · andreicristianpetcu/google_translate_this · GitHub and click google_translate_this_page-1.2.7-an.fx.xpi
Here is how it looks with proxy:
And here is how it looks without proxy
Both transferred and size are different. Basically the browser does not get an unsigned extension, it gets gibberish bytes.