Firefox equivalent of SeaMonkey's pref.js

[Richard Owlett <rowlett@cloud85.net>]

As I date from Netscape era I much prefer SeaMonkey.
However when I did a reasonably default install of Debian 9.8 I got
Firefox 60.5.0esr .

Due to *OPERATOR ERROR*, I mangled a preference.
On the mozilla.general list I was told to edit pref.js .

For SeaMonkey I have no problem.

For Firefox where/what is equivalent file/directory? ?? ??? ???? ??????
TIA