I took a screencast: https://imgur.com/a/xL5CgJ2
With Vimium when you hit "f" it actually appears immediately. It takes a full second with Shortcat.
Firefox (and Chrome) look like giant completely opaque windows to the macOS accessibility API, the only things you can "see" to click on are the close/minimize/maximize buttons, whereas Safari exposes its entire interface and a lot of the web page to the API.
It looks like Homerow is somehow scraping a short list of apps, "Popular non-native apps such as Chrome, Firefox, Brave, Arc, Visual Studio Code, Spotify, Slack, Discord, and Obsidian are supported. I'm working on supporting more non-native apps." I don't know how shortcat etc do it.
[0] https://snap.philsnow.io/2023-04-05T10-04-07.bmgoq2rkwxqmy28... [1] https://snap.philsnow.io/2023-04-05T10-01-44.gdj1z3ukbq712zw...