Nightlies are development releases, they are in constant evolution. Here devs are trying things that will be or not in next beta. Things can be broken anytime. When updating a library to a newer version containing fixes and improvements sometimes long awaited, it happens that things get broken in one or several software depending on it. Sometimes, all you need is to rebuild software depending on that library with the new version. Sometimes software need to be adapted to the new version. Sometimes it’s the new version that is bugged and needs fixing. Every update of this kind, you need to do a lot of testing, and to decide if it’s worthy or not to update. Hopefully, most of time, it goes well and you avoid headaches.
In this particular case, it seems that a newer version of webkit broke videos reading.
Once in a while, devs are deciding that things are stable enough and there has been enough improvements to release something. At this point, they are copying what is in nightlies and are fixing bugs and polishing things in the copy. When they are happy with the results, it can be few months later, they publish this as beta.
Even, if beta also has updates, they are in big majority bugfixes. Devs are paying a lot of attention to not break things and so that the beta remains stable.