Add-on “appears to be corrupt” when trying to install my add-on's .xpi file in Firefox

Question

I am trying to install an extension in Firefox 45 (same happens with Firefox 49), but Firefox does not allow me to do that, indicating that

Answer 1

I had this issue when I compressed the folder containing my addon rather then just the files within.

Don't Compress the Parent Folder of your Manifest

For example, on Windows CTRL+Click on your manifest and any other files or subfolders; right click and select Send to > Compressed Folder and you should be good to go.

Note: from the browser console screenshot we can tell this isn't the issue in this case, but even so this is a common problem to the This add-on could not be installed because it appears to be corrupt error.