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:
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.