Network Buffers and routing tables don’t shrink they grow. Firefox called for them, the Kernel sees the need and allocates them. Since others might need them - they don’t go away for a while sometimes not til the next boot.
Cron jobs also run while Firefox is up - PackKit also uses network resources (the thing that says updates are available) is call by systemd timers.
Run top and iotop to see what happens when Firefox starts.
Probably pages are kept in memory. When you start Firefox again, it will startup faster, because the code pages are still there (the startup time difference will of course depend on how fast your mass storage is, e.g. SSD vs. revolving disk).