Whoa, what a mess!

Is this only on paid video sites like Amazon Prime/Netflix/Hulu -- how do YouTube and other frees look?

Sites like Prime/Netflix/Hulu require the Widevine plugin to decode DRM-protected video. If the problem is only on those sites, I would suspect a problem with the Widevine plugin.

Ref. Watch DRM content on Firefox

I have noticed this with other non-paid streaming sites as well(including youtube). Right now the issue is consistently reproducible in prime video.

I tried disabling Widevine plugin, but then prime says the browser in incompatible. Also checked to see if there are any updates available.

Another aspect is that it is mostly the "Ad" part of the stream that gets this issue and mostly the actual movie/video content is fine. To get through 10-15 seconds of Ad though that gets stretched further(both audio and picture) due to this issue to 30seconds, is a bit troublesome and the timer at times gets stuck rendering the play unusable.

Can you think of any reason your Firefox might treats ads differently than other types of content? There are many content blockers in use, but I can't recall anyone mentioning this particular issue before, so it's hard to know whether a blocker is a factor. ??

I don't use any ad blocker--only 2 addons installed at the moment are:

1. OpenH264 Video Codec provided by Cisco Systems, Inc.
2. Widevine Content Decryption Module provided by Google Inc.

And my extensions would be only

1. IDM Integration Module
2. Surfshark VPN Extension

I would understand if I had recently made any changes, but none--all my settings have been the same for ages. I did uninstall an extension(honey), but that was after the issue had started. I am not sure if any update brought in this issue, but I do not have any other tangible changes to check on.