It's invoking the OpenGL drivers, which seem to be opening a lot of files (a lot of time spent in "open" calls). That indeed has to be intercepted by the sandbox. We are remoting all WebGL operations to get rid of this kind of thing (drivers messing around on the filesystem), so this will be fixed by bug 1638466.
Bug 1679863 Comment 1 Edit History
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It's invoking the OpenGL drivers, which seem to be opening a lot of files (a lot of time spent in "open" calls). That indeed has to be intercepted by the sandbox, forwarded to the parent, checked against a whitelist, and the result sent back. We are remoting all WebGL operations to get rid of this kind of thing (drivers messing around on the filesystem), so this will be fixed by bug 1638466.