OpenGL software rendering is broken on certain Linux systems.
Software rendering with modern Mesa uses llvmpipe, and thus depends on libLLVM.
Depending on which version of the libstdc++ the system's libLLVM was compiled against, the software rendering DRI module (swrast_dri.so) can fail to load, because Tor Browser includes libstdc++.so.6 as part of the bundle (and in the LD_LIBRARY_PATH).
Snipped strace output, with a bunch of env vars set:
Tor/libstdc++.so.6: version `GLIBCXX_3.4.22' not found (required by /usr/lib/libLLVM-3.9.so))This is non-fatal, but webGL will be broken on those systems.