This might be a bit niche, and not specifically JS, but drawing anything in a HTML <canvas> element which involves a CanvaAPI ShadowBlur[1] is guaranteed to make Firefox weep.

The issue was mentioned on Bugzilla (at least) 11 years ago[2], performance remains abysmal.

[1] - https://developer.mozilla.org/en-US/docs/Web/API/CanvasRende...

[2] - https://bugzilla.mozilla.org/show_bug.cgi?id=763120

Idk if being JS heavy was the issue here, but Twitch had terrible performance on Firefox for years. I installed Brave specifically for Twitch (and Twitch alone), and could finally use the site without everything feeling sluggish and super laggy.

I don't use Twitch much anymore, but the one time I opened it recently (for NeoVimConf), it seemed much better.

SAP Concur