Many TVT NVR Camera systems require a plugin to be run in the browser to access their webpage and cameras from a computer. Unfortunately plugins like this are now considered 'dangerous' and so modern browsers no longer have this type of plugin functionality.

 

But all is not lost. The latest Mozilla Firefox version to have this functionality is  51.0.1, 32bit  (since the plugins from the TVT NVR webpage are 32bit):

 

Download from here: https://ftp.mozilla.org/pub/firefox/releases/51.0.1/win32/en-US/  
(Remember to turn off Auto-Update: under Preferences, Advanced, Updater)

 

Or download from Frontmotion, which has auto-update already disabled: https://www.frontmotion.com/news/firefox/releases/firefox-51-0-1/