This works for me even on the latest firefox
How to use Firefox to live view Hikvision device
1. Install the plug in first.
2. Open a blank page of Firefox and input about:config (where you normally type the web address)
3 Press Enter. Click on the, “I Accept the Risk” Button.
4. You will then be in the plugin section of Firefox.
Right click, then go to NEW from the dropdown list. You will have 3 options to choice from, you will need to select Boolean. Once you have done that, you will need to type the plugin name:
plugin.load_flash_only
Click OK and choose “false” for the VALUE.
5. Exit firefox and reload the DVR/NVR web interface to 'accept' to run the plugin at the top of the page (all being well).