No real solutions:

• Clear your cache, clear your temporary internet files
• Pause the video so that it downloads ahead of your playing perhaps 10% then start video, it may be enough to continue to the end without stopping again.
• Get a faster connection, if on Wi-Fi perhaps turn your computer for better signal strength.
• Download the video and play it from the download
• Latest flash (yours is current -- http://www.adobe.com/software/flash/about/ )

Danger Warning: If you do go to the Adobe test page, do not install the download for unrelated software (the big red button), only install from their downloads center link and only the Flash that you need -- do not install "crapware" that may be offered also on your download/installation.

The default of the pref network.http.max-connections has been increased from 30 to 256 in Firefox 6+ versions.

Try to decrease the value of the pref network.http.max-connections from 256 to 30 as used in Firefox 3 versions.

• https://support.mozilla.com/kb/Firefox+never+finishes+loading+certain+websites

See also:

• http://blog.bonardo.net/2011/09/30/is-your-firefor-freezing-at-regular-intervals
• Places Maintenance: https://addons.mozilla.org/firefox/addon/places-maintenance/

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode