When the environment variable SSLKEYLOGFILE is set, Chrome and Firefox are supposed to log ssl information to that file to assist with debugging. After some experimentation, I got it to work in Chrome, but no matter what I do, Firefox won't output anything to SSLKEYLOGFILE.
Note, I am using MacOS
The steps that worked for Chrome are
export SSLKEYLOGFILE="/Users/temp/sslkeyfile"
open /Applications/Google\ Chrome.app/
However, if I run Firefox in the same way, nothing happens, no matter what I try. (I am using the official Firefox build 67.0.2)
After some searching, I found this bug, which suggests that it is an open bug in Firefox which is fixed in Nightly. However, I tried Firefox Nightly (69.0a1 (2019-06-13)) with the same results. No matter what I do, Nightly won't write to the key log either. What can I do?
The SSLKEYLOGFILE was originally disabled when the Mozilla team were debugging an NSS issue in Firefox 65. I had reported the bug here originally. It was subsequently reenabled in Firefox 66. However, once again for Firefox 67 it had accidentally been disabled in release builds again. I once again reopened that original bugzilla ticket to report it. And they then opened up a new bugzilla task that you linked in your post. A recent commit has removed the conditional that should now prevent that bug from reoccurring in future releases. My guess, the SSLKEYLOGFILE env. variable will work again when Firefox 68 releases, and on some Nightly version very shortly.