Firefox Configuration Guide for Privacy Freaks and Performance Buffs
https://12bytes.org/articles/tech/firefox/firefoxgecko-configuration-guide-for-privacy-and-performance-buffs/
Firefox Hardening, disable telemetry etc with custom user.js in the profile directory
The arkenfox user.js is a template which aims to provide as much privacy and enhanced security as possible, and to reduce tracking and fingerprinting as much as possible - while minimizing any loss of functionality and breakage (but it will happen).
https://github.com/arkenfox/user.js
https://github.com/arkenfox/user.js/wiki
Firefox Search Engine Cautions, Recommendations
https://12bytes.org/articles/tech/firefox/firefox-search-engine-cautions-and-recommendations/
Adding search engines to Firefox
Possibly the easiest way to mitigate risks to your anonymity posed by the default Firefox search engines is to simply disable all of them and use alternatives such as the open source and highly customizable Searx meta search engine which you can host on your own server if you like, or you can use any one of a number of Searx instances hosted by others. Like DuckDuckGo, Startpage and others, Searx does not use robots to crawl the web and index content like Google, however the big difference between Searx and most of the other meta search engines is that it is capable of pulling results from many other indexes including Google, Yahoo, Bing, Wikipedia, DuckDuckGo, Startpage, Qwant and more, as well as decentralized peer-to-peer indexes such as YaCy. The Searx interface also offers a lot of configuration options for fine-tuning your search results, including the ability to select exactly what combinations of search engines you want to use for a particular type of search, of which there are currently 10.