Many TVT NVR Camera systems require a plugin to be run in the browser to access their webpage and cameras from a computer. Plugins like this are now considered 'dangerous' and so modern browsers no longer have this type of plugin functionality.
The latest Mozilla Firefox version to have this functionality is 51.0.1, 32bit (since the plugins from the NVR webpage are 32bit):
Download from here: https://ftp.mozilla.org/pub/firefox/releases/51.0.1/win32/en-US/
(Also don't forget to turn off Auto-Update: under Preferences, Advanced, Updater)