Enable hidden option in Firefox first to be able to add custom search engine using URL query.

• Open a new tab and type about:config in the address bar

• Add new setting with name

  browser.urlbar.update2.engineAliasRefresh
  

  With type Boolean and value true

Add the custom search engine to Firefox.

• Go to Settings -> Search -> Search Shortcuts

• Click the new "Add" button in bottom right after the "Remove" button

• Fill the engine name and alias with whatever you want and then in Engine URL part insert:

  https://www.perplexity.ai/search?s=o&q=%s
  

  Then click "Add Engine" and done.

• Make it as default search engine ;)