To start off, this isn't some shoddy laptop or anything. And while it's not the highest of end computers, it's still a gaming computer that definitely shouldn't be getting hit like this. It seems like it's going way too heavy on my CPU, and quite a bit on my RAM.

OS: Windows 10 64-bit

Browser: Firefox

CPU: i5-3570k

GPU: NVIDIA Ge Force GTX 1660 (6gb)

RAM: 16 GB DDR3

For example, if I played Friday Night Funkin'; it'll either act normally for the most part, leading up to either weird as hell freezing, only to wind down at random intervals, or just outright freezes my browser + explorer.exe from the moment I hit the loading screen, making my entire computer unusable for about 1 minute until I have a gap of space to hasily alt+f4 or try to end the tab. Then I have about ~5 minutes of time for my computer to 'recover', and firefox is always lingering in the background.

Is there something I should re-install or adjust or something?

I had similar things with shader demos. One thing is for sure, faulty JS can mess your browser up heavily. I spent hours debugging a bug that just vanished after I restarted the browser, for instance. Other than that, try to get the latest updates for the involved software: the os, the GPU and the browser. Also, try other browsers. I use mozilla based (I think) BRAVE and also, surprisingly: Edge, because hard to believe but Edgne performs best with WebGL. Tho, they say Edge is now based on Chrome... which would be Google - not a fan of monopolies.