In Firefox browser this worked:

If have Firefox, go to page about:addons
Next to Adobe flash, select "Never Activate"
Go to video page where is JW player and reload it (F5)
Right click and View Page Source
Then Ctrl+F and search for .mp4 and .flv
you may be lucky to find download url

Have Chrome or other browser? Try to go into plugins/settings and try to disable Flash. Then reload page where is JW player video and right click and view page source or similar option. Then search for .mp4 and .flv in aim to find URL of the video.

I found no browser plugin/addon that download JW player videos or which disables flash in aim to obtain video URL.