This is typically a port used by a firefox plugin, such as flash and it’s usually handled by plugin-container.exe, which is a firefox process that allows plugins to run in their own address space. Do you have Application rules for plugin-container.exe?
Plugin-container.exe typically won’t load until the first time it’s requested by a plug-in.
There are several entries in about:config that control the behaviour. So, if you don’t want plug-ins loading separately, you change the values for these entries.