Per https://extensionworkshop.com/documentation/publish/self-distribution/#web-download
When the user clicks a link to the signed extension package Firefox should download and ask to install the addon. I was previously using the unsupported InstallTrigger and now removed method to trigger an install.
Since swapping to a plain download link Firefox is only ever downloading the XPI as a file, and never asking the user to install the extension. Is there a specific mime type or response attribute missing that would tell firefox is an addon for install?