I guess it's a regression fix (perhaps Windows-specific) where it used to work and broke at some recent version? Because the feature as described is indeed nothing new in Firefox.
From the linked bug:
> Firefox now supports adding your own custom search engines. Just right-click a search field of a supported website and select Add Search Engine, or go to Settings > Search > Add (below the search shortcuts table) to manually enter a search URL.
From the linked bug:
> Firefox now supports adding your own custom search engines. Just right-click a search field of a supported website and select Add Search Engine, or go to Settings > Search > Add (below the search shortcuts table) to manually enter a search URL.
https://support.mozilla.org/en-US/kb/add-or-remove-search-en...
Mozilla documentation of the feature (updated 2024): https://support.mozilla.org/en-US/kb/add-or-remove-search-en...