Many developers considered version 5.10 the most stable version before the major architectural changes of version 6 and 7.
Searching for is a step backward for any modern web project. The "High Quality" you seek is now standard in any modern HTML5 player without the security risks associated with pirated, legacy software. For a secure and professional site, always choose official, supported versions of video playback tools.
Using nulled versions of JW Player—especially an ancient version like 5.10—is highly discouraged for the following reasons: