Firefox with e10s enabled (all current releases) detects version differences between parent process and and a child process started at a later point in time. Until recently it aborted the entire browser when that happened. I think now they have some logic that tries to keep running with the already open processes and abandoning the incompatible child.

Ideally they'd just prefork a template process for children and open fds for everything they need, that way such a detection wouldn't be necessary.