Firefox 69.0.1 (64-Bit)
Linux Mint 19.1 Cinnamon

I've disabled all add-ons and removed all service workers in about:debugging

When I go the the e621 forum, Firefox makes network requests to mc.yandex.ru
(sorry for not being more precise, I'm a noob)

https://imgur.com/OtfLENT

This thread has been hijacked by Russia Gang.

Meanwhile, I'm going to look into this because I'm bored.

Edit: I get

https://mc.yandex.ru/metrika/tag.js net::ERR_BLOCKED_BY_CLIENT

on Chrome.

It's right in the bottom of the forum index HTML:

<script type="text/javascript" >
   (function(m,e,t,r,i,k,a){m[i]=m[i]||function(){(m[i].a=m[i].a||[]).push(arguments)};
   m[i].l=1*new Date();k=e.createElement(t),a=e.getElementsByTagName(t)[0],k.async=1,k.src=r,a.parentNode.insertBefore(k,a)})
   (window, document, "script", "https://mc.yandex.ru/metrika/tag.js", "ym");

   ym(51832286, "init", {
        id:51832286,
        clickmap:false,
        trackLinks:true,
        accurateTrackBounce:true
   });
</script>
<noscript><div><img src="https://mc.yandex.ru/watch/51832286" style="position:absolute; left:-9999px;" alt="" /></div></noscript>

It is a tracking script, followed by a tracking image that is loaded if the tracking script is blocked.

It is just the Russian equivalent of Google Analytics, since Yandex is the Google of Russia.

https://en.wikipedia.org/wiki/Yandex.Metrica

As of 2019, Yandex.Metrica is the third most widely used web analytics service on the web.

Some people block Google Analytics with a passion, so they are going to want to do the same to Yandex.

My extensions block the script and don't even allow me to load https://mc.yandex.ru/. I doubt the tracking image is successful either.

Google Analytics cut us off because of our traffic volume several years ago so we swapped to using Yandex Metrica, which is what this domain points at. If you do not wish to participate in that, block the domain as you would google analytics.