Hello, go to http://leo.org and let it pronounce any English word. Very often (though not always) the audio playback will end with a rather ugly pop sound. (This is indeed a problem of this port and not of the website. I've tested it on other platforms, such as Debian Linux and Windows.) As you can see below, my sound options are set to ALSA. I haven't tried any of the other options. I've built firefox (both firefox and firefox-esr are affected) with the following options: OPTIONS_FILE_SET+=BUNDLED_CAIRO OPTIONS_FILE_UNSET+=CANBERRA OPTIONS_FILE_UNSET+=DBUS OPTIONS_FILE_UNSET+=DEBUG OPTIONS_FILE_UNSET+=DTRACE OPTIONS_FILE_SET+=FFMPEG OPTIONS_FILE_UNSET+=GCONF OPTIONS_FILE_UNSET+=GNOMEUI OPTIONS_FILE_UNSET+=INTEGER_SAMPLES OPTIONS_FILE_UNSET+=LIBPROXY OPTIONS_FILE_SET+=OPTIMIZED_CFLAGS OPTIONS_FILE_UNSET+=PGO OPTIONS_FILE_UNSET+=PROFILE OPTIONS_FILE_SET+=RUST OPTIONS_FILE_UNSET+=TEST OPTIONS_FILE_SET+=GTK2 OPTIONS_FILE_UNSET+=GTK3 OPTIONS_FILE_SET+=ALSA OPTIONS_FILE_UNSET+=PULSEAUDIO OPTIONS_FILE_UNSET+=SNDIO
Before investigating find the affected audio then paste URL here from Hamburger Menu -> Developer -> Network (Ctrl+Shift+Q) -> Copy -> Copy URL. Can you reproduce outside Firefox? Can you find the popping fragment via an audio editor? Have you tried to lower volume of everything but vol/pcm via mixer(8)?
It seems the issue is no longer present in more recent versions of both www/firefox and www/firefox-esr. :-)