Whats weird is if i rewind it freezes and if i fast forward it catches up and corrects it..never seen this before. I tried installing ffmped-6 but that didnt help either.
Also - the same videos WORK on Chromium. Tried Firefox in safemode/extensions disabled and same thing.
You should stick with Chromium. I use Chrome and it's way faster and better on Zorin OS. If your PC has low RAM, this problem may appear. So, it's better to upgrade your RAM. Switching from HDD to SSD or keeping both will help. Or if you don't want to upgrade your configuration, don't keep too many apps and tabs open. Close unnecessary ones.
I would suggest Microsoft Edge (Even Edge is based on Chromium) because they have better ram management and performs better with low ram.
Try disabling all the extensions and close the background apps and keep only 1 tab open incase you don't wanna switch to other browser.
Upgrading the RAM is a really good solution for you if your laptop have low RAM.
SSD may help you in Faster boot times, opening apps (Even RAM helps on opening apps faster) faster etc.
I would ask you to give you PC specifications.
I must agree with @Magnibigg and @DarkFuture above. I use an ungoogled Chromium browser and for this very reason. I was always a FireFox user in the past.
But only the Chromium based browsers give the performance that I need and the behaviors that for me are the most useful. For example, with my browser, if I have multiple tabs open; tabs that sit unattended get put to sleep rather than having them continue to gobble up RAM.