I had a look, no errors here when starting Firefox from the console (mozilla-firefox-91.3.0esr-x86_64-2).
I get this on shutdown, but it doesn't look related:
Code:
###!!! [Child][RunMessage] Error: Channel closing: too late to send/recv, messages will be lost
[Parent 5993, IPC I/O Parent] WARNING: FileDescriptorSet destroyed with unconsumed descriptors: file /tmp/firefox-91.3.0/ipc/chromium/src/chrome/common/file_descriptor_set_posix.cc:19
###!!! [Parent][RunMessage] Error: Channel closing: too late to send/recv, messages will be lost