I am developing an Add-on and I am having trouble finding documentation that explains how to let Firefox know where to look for updates. If I post an .xpi on my website and the user downloads and installs that .xpi, does Firefox know to check that URL for changes? or do I have to put something in package.json?
Are you using the online builder or the SDK command-line tool? You can only specify the update url using the cfx tool:
https://developer.mozilla.org/en-US/Add-ons/SDK/Tools/cfx
When building your add-on, you would run cfx xpi with the following arguments:
cfx xpi --update-link https://example.com/addon/latest
--update-url https://example.com/addon/update_rdf
If you have already built your add-on using builder, a hackier option would be to download the xpi, unzip it, manually edit install.rdf and then re-zip it. See MDN for documentation on install.rdf syntax: