Firefox doesn’t explain how to do this at all, bjt it is possible. Make a bookmark with the URL you want, and set the keyword to whatever symbol you want ti start it with.

For example,

• Name: Scryfall (or whatever you want)
• URL: scryfall.com/search?q=%s&unique=cards&as=…
• Keyword: s Then type “s Birds of Paradise” to get the result you want.

I did the same with Reddit and it worked on my end. If it doesn’t work for you I’d be happy to help you figure it out.