~1 week ago. It had been an ongoing issue for the past few months.

On which hardware?

MacBook Pro (15-inch, 2017) 3.1 GHz Quad-Core Intel Core i7 16 GB 2133 MHz LPDDR3

I have approximately the same Macbook (but from 2015), one issue i have is playing 4K .webm videos on Youtube. It doesn't matter which browser, it even happens when i download the video and play it in another player. My CPU will spike and the fans start blasting.

4K videos in mkv/H.264 format i can play fine though.

The issue here is the hardware decoders capability;

Depending on which browser you use, and whether your Macbook is connected to power, websites will send you a different video/codec/size.

Here's a random post that sort of describes this: https://apple.stackexchange.com/a/409764

It works fine in another browser. So I don't think it is a hardware issue? Is there a situation where Chrome/Vivaldi would be able to use hardware the FF wouldn't? And 90% of my time is spent on power (top left usb-c port).

Using a different browser results in a different video (/codec) being downloaded. The different codec is what can cause your hardware/cpu to spike.