You'd be surprised. When working on Firefox I regularly run into files with "Copyright 1999 Netscape Communications..." file headers.

How much of NSPR is left in there? It's been a long time since I looked at the codebase.

NSPR is still all over the tree. Off the top of my head there's some log functionality, the print format strings, and the environment variable support in use. Searchfox turns up a bunch more: https://searchfox.org/mozilla-central/search?q=PR_&case=true...