doc: fix Firefox instructions for adding Sourcegraph search engine

Created by: slimsag

These are heavily outdated and broken today (a customer reported it). I tested these updated instructions on the latest FF.

Rendered: https://github.com/sourcegraph/sourcegraph/blob/8271f20537c757c866e9ea90b7635ab4710c5191/doc/integration/browser_search_engine.md#firefox

Fixes #5627