Firefox does not generate SSLKEYLOGFILE when the variable is exported.
I'm running firefox in headless mode.

The problem is present for both firefox and firefox-es on Debian
unstable and testing,
for at least amd64 and arm64 architectures. Debian stable generates
SSLKEYLOGFILE as expected.

To reproduce, build the following Dockerfile, with
```
docker build --build-arg=CACHEBURST=$(date +%s) --progress=plain --tag
sslkeylogfile .
```

Dockerfile
```
FROM debian:unstable

RUN apt-get update && apt-get install -y firefox-esr
ARG CACHEBURST=0
RUN apt-cache policy firefox-esr
RUN dpkg -l firefox-esr

RUN export SSLKEYLOGFILE=/tmp/SSLKEYLOGFILE && echo SSLKEYLOGFILE is
empty > /tmp/SSLKEYLOGFILE && firefox --version && firefox --headless
--screenshot /tmp/screenshot.png https://www.google.com && echo
$SSLKEYLOGFILE && cat $SSLKEYLOGFILE

CMD sleep infinity
```

Output (from Debian unstable)

```
#7 [3/5] RUN apt-cache policy firefox-esr
#7 1.394 firefox-esr:
#7 1.397   Installed: 115.13.0esr-1
#7 1.397   Candidate: 115.13.0esr-1
#7 1.397   Version table:
#7 1.397  *** 115.13.0esr-1 500
#7 1.401         500 http://deb.debian.org/debian unstable/main amd64 Packages
#7 1.401         100 /var/lib/dpkg/status
#7 DONE 1.4s

#8 [4/5] RUN dpkg -l firefox-esr
#8 0.548 Desired=Unknown/Install/Remove/Purge/Hold
#8 0.548 | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
#8 0.548 |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
#8 0.548 ||/ Name           Version       Architecture Description
#8 0.548 +++-==============-=============-============-============================================================
#8 0.548 ii  firefox-esr    115.13.0esr-1 amd64        Mozilla Firefox
web browser - Extended Support Release (ESR)
#8 DONE 0.6s

#9 [5/5] RUN export SSLKEYLOGFILE=/tmp/SSLKEYLOGFILE && echo
SSLKEYLOGFILE is empty > /tmp/SSLKEYLOGFILE && firefox --version &&
firefox --headless --screenshot /tmp/screenshot.png
https://www.google.com && echo $SSLKEYLOGFILE && cat $SSLKEYLOGFILE
#9 0.589 Mozilla Firefox 115.13.0esr
#9 0.644 *** You are running in headless mode.
#9 0.943 [GFX1-]: glxtest: libpci missing
#9 0.943 [GFX1-]: glxtest: Unable to open a connection to the X server
#9 0.943 [GFX1-]: No GPUs detected via PCI
#9 0.943
#9 4.129 /tmp/SSLKEYLOGFILE
#9 4.130 SSLKEYLOGFILE is empty
```

Expected output (taken from Debian stable):
```
#7 [4/6] RUN apt-cache policy firefox-esr
#7 1.176 firefox-esr:
#7 1.176   Installed: 115.13.0esr-1~deb12u1
#7 1.176   Candidate: 115.13.0esr-1~deb12u1
#7 1.176   Version table:
#7 1.176  *** 115.13.0esr-1~deb12u1 500
#7 1.176         500 http://deb.debian.org/debian-security
stable-security/main amd64 Packages
#7 1.176         100 /var/lib/dpkg/status
#7 1.176      115.12.0esr-1~deb12u1 500
#7 1.180         500 http://deb.debian.org/debian stable/main amd64 Packages
#7 DONE 1.2s

#8 [5/6] RUN dpkg -l firefox-esr
#8 0.666 Desired=Unknown/Install/Remove/Purge/Hold
#8 0.666 | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
#8 0.666 |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
#8 0.666 ||/ Name           Version               Architecture Description
#8 0.666 +++-==============-=====================-============-============================================================
#8 0.666 ii  firefox-esr    115.13.0esr-1~deb12u1 amd64        Mozilla
Firefox web browser - Extended Support Release (ESR)
#8 DONE 0.8s

#9 [6/6] RUN export SSLKEYLOGFILE=/tmp/SSLKEYLOGFILE && echo
SSLKEYLOGFILE is empty > /tmp/SSLKEYLOGFILE && firefox --version &&
firefox --headless --screenshot /tmp/screenshot.png
https://www.google.com && echo $SSLKEYLOGFILE && cat $SSLKEYLOGFILE
#9 0.672 Mozilla Firefox 115.13.0esr
#9 0.728 *** You are running in headless mode.
#9 1.039 [GFX1-]: glxtest: libpci missing
#9 1.039 [GFX1-]: glxtest: Unable to open a connection to the X server
#9 1.039 [GFX1-]: No GPUs detected via PCI
#9 1.039
#9 4.387 /tmp/SSLKEYLOGFILE
#9 4.387 SSLKEYLOGFILE is empty
#9 4.387 CLIENT_HANDSHAKE_TRAFFIC_SECRET ...
...
```