I’m not sure why the OSC plugin for JOSM is really buggy on my machine (Xubuntu 18.04.4 LTS) so I browsed its website instead.

But sometimes the website itself dragged Firefox to crawl and once in a while, freezes the whole system (but I was able to get back to editing without losing data). Mapillary never did something like this.

So I disabled the hardware acceleration on Firefox. Looks like it’s working well so far.