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Click the Open Menu at the top-right of the Firefox page, and click Add Ons.
Expand title Click here to view a screenshot... - Click Extensions.
- Find Dictionary Extension, and click Disable.
Click the Open Menu at the top-right of the Firefox page, again, and click Preferences (Options for Windows).
Expand title Click here to view screenshots... OS X, macOS Sierra
Windows
- Click Privacy.
- Uncheck Request that sites not track you and Use Tracking Protection in Private Windows.
- Restart Firefox and resume the testing session.