I think the Firefox thing might actually be KDE restoring your session and not Firefox. Try to check KDE settings to see if you can start with a blank session on login.

Check out flameshot, not quite the same but still very good.

Can’t you setup Firefox to remember the last session and reopen the same pages upon relaunch without any user interaction?

I feel like Linux is good for power users or users like your grandfather who are not very tech savvy. The issues arise with users savvy enough to get themselves into trouble but not enough to understand how to fix it or how to do a slightly more advanced task.