The Waterfox source code is a specialised modification of the Mozilla platform, designed for privacy and user choice in mind. You should be able to install it and compile Waterfox without any issues. Other modifications and patches that are more upstream have been implemented as well to fix any compatibility/security issues that Mozilla may lag behind in implementing (usually due to not being high priority). High request features removed by Mozilla but wanted by users are retained (if they aren’t removed due to security). Waterfox focuses on giving users choice. The browser is focused on power users, which lets you make the important decisions. There is no plugin whitelist (meaning you can run Java Applets and Silverlight apps), you can run whichever extensions you like (including bootstrapped add-ons that can completely change functionality of the browser) and absolutely no data or telemetry is sent back to Mozilla or the Waterfox project. Browse the web your way, with the indie web browser: No Telemetry, Limited Data Collection, NPAPI Plugins, Bootstrapped add-ons, Polish your userChrome, Stable API.
Like Firefox, Waterfox wants people’s attention since their default search engine is paid for by the company that bought them a few months ago https://www.waterfox.net/blog/waterfox-has-joined-system1/ – System1 https://system1.com is specialized in advertising. Basically they are buying small search engines or browsers to inject their own search engines into them as default. Yes, anyone can fork this project since it is Open Source. Yes anyone can change the default search engine. But yes, Waterfox is trade-based since they clearly want people’s attention. Else they would have chosen the best search engine they know off, not one that is paid for. Don’t forget that for one search is an essential part of a browser, and second that most people probably do not know how or simply do not change their default search engine.