Sorry for the late reply.
I hope you don’t mind, I signed it and uploaded it. I guess if you uploaded here where it is public I could also upload my signed version. I will take it down after you downloaded it yourself for inspection 
First of all your addon is not signed. You can see this if you unzip my version and look at the contents. Mine has META-INF folder with 3 files. That is the signature. Yours does not have it.
Mine has something different in manifest.json. I removed strict_min_version. That thing is the minimum Firefox version and I guess it should be the minimum depending on what APIs you are using. I think it’s best to leave it out 
Remove it from your aws update.json also. I also added a new GUUID since I do not own your addon and you can only sign your own addons. AMO sees new GUUID and it thinks it is different
To recap, what you need to do:
- remove strict_min_version from manifest.json and from update.json
- Upload your addon to this URL. You need you AMO accont. No account, no signing.
- After your addon gets uploaded and there are no errors, select at the bottom “all platforms” so it will appear in AMO regardless of the user OS (I’m guessing there is nothing OS specific inside)
- Press sign, wait a little and the download your addon. Obviously your version that you uploaded is not signed, the downloaded one is singed.
- Publish it on some web site that “Content-Type” header to “application/x-xpinstall” if you wish your users to install it with one click. Github does this by default for XPI files in the release section
Hope it works out for you!
Cheers,
Andrei