Adding Brave Search to Firefox so that it is available in the default search options isn’t immediately obvious, since it’s not listed under the search providers under Settings->Search
And while there are a number of add-ons which provide it available, none are provided by Brave themselves.
That said the solution is surprisingly simple:
- Start by going to https://search.brave.com
- Then right-click the address bar
- On the menu displayed, select “Add Brave Search”
- Now open “Setting”, select “Search” and choose “Brave Search” under “Default Search Engine”