Found in

• Firefox Nightly 115.0a1 (2023-05-22)

Tested platforms

• Affected platforms: macOS 11, macOS 12 ARM
• Unaffected platforms: Windows 10, Ubuntu 22

Preconditions
media.gmp.decoder.enabled=true
media.gmp.decoder.preferred=false
media.gmp.decoder.multithreaded=false

Steps to reproduce

1. Launch browser with the preconditions set and navigate to about:logging.
2. Set New log modules to timestamp adding: GMP:5,GMPVideoDecoder::Decode,GMP Encode.
3. Set Logging to a file, open directory and click the Start Logging option.
4. Go to https://jsfiddle.net/jib1/02j8q6h9/show and click run and gum!.
5. Return to about:logging and click to Stop logging.
6. Check the mozlog file for GMP encoder and decoder messages, search for "openh264".

Expected result

• At step 4, the video/details correctly appear at the Receiver.
• At step 6, the "openh264" correctly appears in mozlog file ("gmp-gmpopenh264/2.3.2" mean that the plugin its in use and working).

Actual result

• At step 4, the video/details appear intermittently at the Receiver: sometimes the video/details appear correctly, sometimes 0x0, 0 bps is displayed at the Receiver.
• At step 6, the mozlog file is empty (0 bytes), no log is recorded.

Notes

• The issue is not reproducible on Windows and Ubuntu.
• At step 6, we managed to record logs only once on mac 11, on mac 12 ARM we did not manage to record logs at all.
• With the old plugin version (1.8.1.2), on Nightly 113, everything works ok.
• The issue is intermittent, we have not reproduced it constantly in our testing.

Regression range

• New feature implementation.

media.gmp.decoder.preferred=false

Has it really been false? If it's false, you are likely testing Mac's H264 decoder and not GMP-OpenH264.