This is not normal, something's broken with Firefox in your setup. Probably worth filing a bug.
(Also it's almost certainly nothing to do with the JS engine.)
The 'cryptojunk' facilitates funding content sans our traditional advertising dystopia. Whether that's you're cup or tea or not neither the rationale nor the mechanism is difficult to understand. The cryptojunk is also off by default[1].
Should you manage to overcome this hangup and give it a try you'll have a browser that is almost indistinguishable from Chrome except that the bulk of Google's abuse has been removed, among other benefits.
I'm curious why Edge isn't an option for you? I've started using it recently and so far it's a fairly decent experience. Sure, it's Microsoft and run into the same trust and privacy issues you get with Google Chrome, but that being said, my browsing habits are pretty benign so I just accept it for what it is. Let's be honest, options are slim when choosing a web browser.
Another browser on my radar is Valvadi [https://vivaldi.com/], another Chromium based browser with a ton of bells and whistles, most of which can be turned off. It has very much an Opera vibe.
[0]: https://arstechnica.com/gadgets/2021/12/microsoft-edge-will-...
I've been using Firefox as my primary browser for about 2 years now - and the javascript engine on it has recently... sh*t the bed (I cannot paste links or images into facebook, I cannot paste text with line breaks into twitter, I cannot paste using reddit's "markdownmode").
I use google-chrome for netflix, recently chromium stopped working for netflix (apprently the browser is no longer supported)
Edge isn't an option for me, brave isn't on my radar (WHY include cryptojunk with a BROWSER???, it's like the early 2000s where a major vendor was trying to claim that the browser is tied so closely to their kernel as to make it a part of an OS)
Is Opera any good?