Sometimes at least in the span of latest 3 weeks JavaScript fails to load entirely for me in Russian Wikipedia (maybe the error persists in other projects, I can’t say) using latest Firefox, with only this in console log:

Loading failed for the <script> with source “https://ru.wikipedia.org/w/load.php?debug=false&lang=ru&modules=startup&only=scripts&skin=vector”.

I can reproduce this (right now) even with safemode=1. There’s nothing in network tab of DevTools that could indicate why this happens, and disabling or clearing the cache also doesn’t help. The problem concerns all JavaScript and not just sitewide scripts.