On Sun, 2020-12-27 at 08:12 -0500, Hendrik Boom wrote:

> When I
>    killall firefox-esr
> it often (but not always) immediately respawns, reopening all the
> tabs.

>
> Sometimes I have to killall it three times for it to stay dead.
>
> Is there anything I can do to fix this? So it dies and doesn't
> immediatly respawn, but only when I ask for it to resurrect itself?
>
Why not just close it using Firefox's own Close option (File>Quit, or
Ctrl-Q)?
Whether tabs are preserved when you close is an option in Preferences
(Edit>Preferences): it's the first option.
If you close a Firefox window it also usually asks you if you want to
keep its tabs.

--
Marjorie