Here are the official installation instructions for dissenter for firefox
1 Download the extension and unzip it. Find "dissenter_extension_firefox_v015" folder.
2 Go to about:addons in the URL bar.
3 Find the small gear icon within the Addons page. Click it the gear button. Find "Debug Add-ons" from the dropdown menu. Select that option in order to navigate to this page: about:debugging#addons.
4 Click on the "Load Temporary Add-on..." button in the top right of the extensions page.
5 After clicking "Load Temporary Add-on...", the file opener opens, select the extension's manifest.json file within the "dissenter_extension_chrome_v015" folder, then click "Open".