Sorry, I could’ve sworn KDE also allows you to save sessions. 
My autostart is empty also because i don’t have anything set to load on startup. Your desktop session though looks greyed out? Not sure if that’s the case but that’s what the image looks like. Here is mine.
As @BONK said is best to pin tabs in Firefox. You don’t want Firefox to be loading on startup. It is one app that takes the most memory. Every tab that opens adds more memory. Bad idea!
Or as @keybreak said go into Firefox and set restore previous session.
It does but it is in a different location under settings, system settings, startup & shutdown, desktop sessions.
How to manually save session ?
I believe “restore previous session” is not a good idea. because i experienced in windows os before , when the previous session is a broken session (something causes freeze of whole os), then everytime reboot the os, the system will be freezed, unless safemode is activated (or live session).
how to save the session ?
Thx
On mine the two top boxes are checked first of all.
Confirm logout
Offer shutdown options
Then the centre box won’t be greyed out maybe because currently it is set to end current session. Mine is set to restore previous session in the bottom box.
I disabled confirm logout and offer shutdown options… i thought those are the feature that causing me to click twice to shutdown pc… i want my laptop to shutdown when i click shutdown once… previously when i clicked “shutdown” it will ask me for confirmation to shutdown… that’s one feature i dislike…
Maybe i recognition of “confirm logout” and offer shutdown options were wrong ?
Mine are all set default. I don’t really have any shutdown issue. Mine is a desktop and it shuts off instantaneously mostly. If i leave any open tabs in Firefox they reopen where they were i think.