Is there any code in Firefox (as of 2020) that comes from Netscape Navigator?
Server side, so not in Firefox, but I personally got a lot of mileage out of NSAPI.  For the company I worked at, at the time, it was the only way to do something better than old-school, fork a process per request, CGI.  We had persistent socket processes handling web requests, in parallel, WAY before things like mod_perl or FastCGI. As I remember, it was there around 1995 or so.  Many thanks to any former Netscapers.  For comparison mod_perl was 1999 or so. FastCGI was really beta and unknown (as I remember) in late 96.