Reasons are the data transmissions happening by default and Mozilla's questionable inclusion of add-on things like Pocket. See for example:
https://www.kuketz-blog.de/mozilla-firefox-datensendeverhalten-desktop-version-browser-check-teil20/
vs.
https://www.kuketz-blog.de/librewolf-datensendeverhalten-desktop-version-browser-check-teil8/ and https://www.kuketz-blog.de/mullvad-browser-datensendeverhalten-desktop-version-browser-check-teil22/
You might need to translate the site to English. If you compare that, you can see why it's easy to recommend the forks over the original. That said, you CAN configure Firefox to also behave well, but that takes an extra effort. It is far from there by default.