I was following James’ Search engine shortcuts in Firefox which was very helpful, though I was not seeing this part of step 5 in Firefox on Windows:

“Click the icon of the site in the ‘This time, search with:’ bar. If you haven’t already added the site as a search engine, a small green + icon should appear indicating you can add the site as a search engine.”

I knew that indieweb.org advertised OpenSearch, so I thought there should be a way the browser picks up on that. I found that right-clicking in the address bar gave me the option Add “IndieWeb (en)”.

That worked and the search now shows up under Settings > Search Shortcuts. However, it appears that the edit option is disabled, which seems like it might be a bug. That prevents me from adding a search shortcut like @iw unless I manually add the search engine.