So we all know that "Stop downloading" was the way of viewing the thumbnail back in the day, makes you feel old huh. Anyway short story short: This is how you do it nowday.
Read first: This is how to do it on Google Chrome, I do not know if this works for any other web browser but if it does feel free to update me at any time, much appreciated :)
1: Click on video
2: Press Ctrl + U
3: press Ctrl + F and search for thumbnail
4: Scroll down to the last ones
5: Press the .jpg links and bam.
If you didn't catch that. Here's the long version ^.^
1. Click on the video in which you want to view/download the thumbnail 2. You need to have the video window selected when doing this so pause the video and then start it again just to make sure. 3. Now without pressing outside the video or on any other spot than the video window box; press on the keyboard: [Ctrl + U] - If done correctly a window with alot of "what the hell is this" will pop up, if not you've done something wrong above. 4. Now it might seem a bit tough to look at all these number and letter codes but all you have to do is to open the word searcher, press: [Ctrl + F], now a small search bar will pop up in the top right corner (if you have Chrome, not sure on the other engines) and in the search bar you want to search for what you're looking for: "thumbnail". Now when doing this you'll be automatically put on the first thumbnail word that is on the whole code page, (there are usually 5 thumbnail words) You want to look for the last one by looking at the scroll bar furthest to the right you can see the yellow lines corresponding the word you searched for, in this case the "thumbnail" word. Scroll down untill you are on the last one. It will look something like this:
Now you can just press any of the links that is ending with .jpg next to the hightlighted thumbnail's and whaaam! you got yourself the thumbnail in a pop up window, ready to be downloaded.