If you are just trying to search a certain site from the address bar, you do not have to create any OpenSearch XML.
You can add a bookmark with %s as a placeholder, and give it a keyword. The subsequent search terms will fill into the placeholder. For an example that searches Stack Overflow, put these fields into a “New Bookmark:”
| Field | Value |
|---|---|
| Name | Search Stack Overflow |
| Location | https://stackoverflow.com/search?q=%s |
| Tags | development (your relevant tags) |
| Keyword | so |
| Description |
To perform a search, type so How to exit Vim into the address bar and hit return.
If you have also added search engines in Chrome, you will recognize the same %s placeholder that is used in chrome://settings/searchEngines. I do not know, but it may be that Chrome also uses bookmarks like Firefox under the hood.