Firefox now supports drm, but I've heard it has some risks, Do you know if it's safe? Maybe it executes in a sandbox or something like that.
closed as unclear what you're asking by mikewhatever, Anwar, Zanna, Eric Carvalho, Pilot6 Sep 23 '16 at 13:05
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DRM (or, Digital Rights Management) is a way for content producers to secure their content from unauthorized viewers in a relatively reliable way.
DRM, by nature, will grant no inherent security flaws to your system as it's just a way to ensure that you are able to read content. Now, this may mean that DRM checkers may run some limited code on your computer to make sure that you're actually permitted to view the protected content. Worst case, a so-far unknown vulnerability in the DRM check system could be used as a drive-by malware attack, but this risk is no less present than it it in advertisements and the like.
The main opposition and the "risk" of DRM (as implied by your comments) isn't a security issue, but the risk to free media and culture, as it can be used to limit freedom to some extent. However, DRM checking is no more dangerous that already-existing web tools, and it really doesn't have any more power over your system.
If you are uncomfortable with it for whatever reason, it's always possible just to disable DRM enforcement, which may have the side effect of blocking your access to some web content. To do this, simply disable any DRM-related plugins in Firefox (likely just the Adobe Primetime DRM plugin).
In your specific case (Firefox), you're using a platform known as the aforementioned Adobe Primetime DRM system. Essentially, this is a glorified password manager meant to ensure that you have secure access to a protected stream by way of encryption. As long as there are no major security vulnerabilities in Primetime, it will do no more than decrypt media for you to watch whenever you request DRM-protected content.
Its not really possible to answer this as the DRM used is closed source. We can't really know how it works without reverse engineering it which would be illegal because of DMCA.