I have noticed that there is a plug-in called Widevine on Mozilla. I had assumed that plugins were a security concern and have been deprecated of for a long while. I also have NO notion if I ever need its function, or is critical to a browser, or in between.

1.Are there guidelines (for complete non-expert) to decide whether it is needed?

Since the publisher Google is known as anathema to everything privacy and safety:
2. Is there an open source alternative with acceptable functionality?