Been experiencing this since using your docker image. Finally found a fix its because the default /dev/shm size is 64mb (df -H /dev/shm to check size).

The fix is to add -v /dev/shm:/dev/shm so it maps docker host memory across. Or you can add a specific amount of memory using --shm-size 2g for example. Would be great to get an environmental variable or fix included/update documentation to reflect.

Full info on fix on https://hub.docker.com/r/jlesage/firefox#increasing-shared-memory-size