This is talking about webrender's on-disk shader cache for the shaders it itself is deciding to cache, not your GPU driver's shader cache.

Looking at https://searchfox.org/mozilla-central/rev/adcc169dcf58c2e45b... it looks like that error means the disk cache was not created.

So either get_cache_path_from_prof_path failed to return a path or create_dir_all failed.

I haven't dug through more details, but I'm pretty sure the shader cache code there is new and am not at all sure it's completely hooked up yet.