Until it will be fixed in some next release of SeaMonkey users can't download "assets" from GitHub websites. I'm just reminding there is simple workaround for this problem.
Example:
PDF Viewer extension:
https://github.com/IsaacSchemm/pdf.js-seamonkey
To download click on the right "PDF Viewer 2.3.235 Latest". New webpage will have such address:
https://github.com/IsaacSchemm/pdf.js-s ... g/v2.3.235
User can't see download links because some script(s) gives spinning circle only.
The trick is to replace "tag" in URL (at Address Bar) with "expanded_assets" which will give such link:
https://github.com/IsaacSchemm/pdf.js-s ... s/v2.3.235
Then user can click and download XPI or EXE or whatever in other cases will be there.