A quick note on why and what: due to the nature of existing intellectual property controls and the resulting need for certain protections to be in place (and the resulting lack of protections in place for OSS) there is a ‘facefork’ of the mozilla suite (that’s mozilla browser, firefox browser, thunderbird e-mail client, etc.) into a series of virtually identical builds with only branding changes (hence “face” fork).
The internals are the same and aside from minor patching will remain the same. The impetus is simply removing restricted IP to allow timely patching. In order to protect the mozilla name they wanted controls in place on patching and needed to have some restrictions on logo/name use. Therefore those that want freedom have to have a layer of isolation. Nothing to be ashamed of and no hard feelings. Software marches on.