Firefox Private Mode
You cannot run add-ons in Firefox Private Mode using the normal side-loading technique. This is
because only they appear in the about:debugging page, which doesn’t give you the option to enable
the extension in Private Mode.
As an alternative, you can install the development build as an unsigned add-on using the steps below.
Prerequisites
- 
Install Firefox Developer Edition 
- 
Configure Firefox Developer Edition: - Open Firefox Developer Edition
- Navigate to about:config
- Set the xpinstall.signatures.requiredsetting tofalse(add the setting if it’s not in the list already)
 
Steps
- Open your local browser repository on the command line
- Build and package the browser extension with npm run dist:firefox
- Open Firefox Developer Edition and navigate to about:addons
- Click the cog in the top-right next to “Manage Your Extensions”
- Click “Install Add-on From File”
- Open dist/dist-firefox.zipin your local repository
- The extension will now be the about:addonspage. Click on the name of the extension to expand it, and then toggle “Run in Private Windows”.