See https://sql.telemetry.mozilla.org/queries/72658/source#182143

This looks like a case of PR_LoadLibrary() failing due to the new linker cache and dynamic library behavior in Big Sur documented below. More specifically, the call to PR_LoadLibrary() in CGLLibrary::EnsureInitialized() is failing to load /System/Library/Frameworks/OpenGL.framework/OpenGL because the access(2) call in PR_Access() returns -1.

From https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11-beta-release-notes/

New in macOS Big Sur 11 beta, the system ships with a built-in dynamic linker
cache of all system-provided libraries. As part of this change, copies of dynamic
libraries are no longer present on the filesystem. Code that attempts to check
for dynamic library presence by looking for a file at a path or enumerating a
directory will fail. Instead, check for library presence by attempting to dlopen()
the path, which will correctly check for the library in the cache. (62986286)

I'll try a patch that skips the access() check and report back the results.