When you open a video in YouTube, the flash player plays the flv file. If you want the high definition version, click on the HD button presented at the bottom of the flash player in your browser. The HD version will be loaded up. The HD version is more in size and is better in quality too.
How to disable all the flash in SeaMonkey using AdBlock plus ( flashblock not working when JavaScript is disabled)
Flash block is a very good add-on for blocking flash in SeaMonkey, but when you disable JavaScript in the browser, the flashblock add-on does not work. So here is an alternative. In SeaMonkey after installing AdBlock plus, do the following to remove flash completely. Open AdBlock plus (Go to tools > AdBlock plus). Now click the filters menu and select ADD FILTERS. Now in the filter rule type *.swf and click the apply button. Here after you will not be able to view flash in your browser. Blocking flash helps save a lot of bandwidth. If you want to see flash in a particular page then you will have to disable AdBlock plus by going to AdBlock plus > options >and uncheck the Enable AdBlock plus menu.