Not sure this is right forum, but here goes.
Just did a fresh install of Mint 19 after running 17.3. Now when I play YouTube videos, the actual video does not change when switching to another video file. The webpage shows the all the info for the newly selected video file, but the actual video in the playback window is still the previous played video. You must manually refresh the page for the correct video to show up matching the displayed webpage. I thought it might be a browser issue(I'm using Firefox with AdblockPlus) but I installed Vivaldi with no blockers, etc. and it does exactly the same thing. I'm mystified about what is happening, or where to begin to correct it.

Format of video file ? (file type)

Please read what the OP said - I highlighted it in bold for you. YouTube videos the last time I checked use Flash videos.

@1bSpanMan,

When I first read your question, I was thinking it's probably a issue with flash player. While it could also be a browser issue, my guess is a flash issue.

It's possible that Firefox extensions maybe causing the problem. Try running Firefox with all extensions disabled with the command below and see if you still get the same issue. Then report back.

My understanding, based on random stuff I've read and heard and cannot cite, is that YouTube has been trying to migrate away from Flash for some time.

According to Wikipedia offers videos in several different viewing formats, including FLV, H.264, and 3GP for mobile phones.
https://en.wikipedia.org/wiki/YouTub...ty_and_formats

Sorry to take so long to reply. I am still having the same issue after restarting Firefox in safe mode. I should note that not every video clip hangs up, only the large majority. Even with Firefox running normally and with ad blockers active perhaps 20% of the video clips will load the first time they are selected. I haven't been able to see a pattern as to which loads and which hangs to the previous clip. As I mentioned in original post I downloaded the new Vivaldi browser without any adblocking extensions and it too hangs to the last clip. When you select a new clip, the page loads to display the chosen clip but the prior clip is still displayed in the playing window. Refreshing the page brings up the correct clip. I haven't noticed this happening on other websites, but then You Tube is the only site I visit where I watch more than one or two clips at one sitting.