Startup applications - a casual query

I am running Peppermint 3 on a Dell D505 laptop.
Earlier this year I begged help in adding Firefox & Thunderbird to the startup list via the config file. Both applications then appeared in the Menu/Preferences/Desktop Session Settings.
Although everything is working well, I find that Thunderbird is no longer listed in the …/Session Settings, and unticking the Firefox entry does not stop it loading.
Is this because the startup config file over-rides it? I don’t really mind as I can remove items from the config file if necessary, but it would be nice to know.
This query is a prelude to a more interesting post about start-up. One thing at a time for me!

I’m not 100% sure the “desktop session settings > startup applications” menu item is fully functional in Peppermint 3 (can’t remember).

From what I gather there are 2 directories where adding a .desktop file adds it to that list

/etc/xdg/autostart
(system wide)
and
~/.config/autostart
(user specific)

IIRC, only ones added to the former can be turned on/off in the list

Hmm. I know I used the second of those. Anyway, it’s not a real problem as I know how to un-autostart them.
BTW: IIRC?

IIRC = If I Remember/Recall Correctly :wink: