After further testing I discovered that in order for 360 browser to function properly it requires the unofficial Service Pack 4
AND the Post SP4 Update Pack. I already had SP4 but not the Update Pack.
You can download SP4 + SP4 Update Pack from here:
https://ryanvm.net/forum/viewtopic.php?t=10321
The Update Pack apparently updates root certificates as well as certain XP system files that enable 360 Browser to function properly.
Here is 360 Browser after applying the above mentioned updates..
Youtube now works perfectly - thumbnail previews, playback, even hovering over videos plays.
Bitchute still doesn't work. I'm not a fan but I know some people here are.
Amazon, which normally runs slowly for me and occasionally freezes with all XP Mozilla browsers is quick and responsive.
Skype for Web, previously unavailable in any XP browser works in 360 Extreme.
Zoom for web wouldn't work with video, only audio but I think Zoom still makes a desktop client for XP.
All in all this browser looks very promising. I would still get certificate errors with some sites but looks like 360 Extreme Explorer is a viable alternative to Mozilla browser in XP. I'll keep testing and compare which sites work better than Mozilla browsers.
Download it for yourself and compare - just remember the requirements at the top of this post.