Firefox has a built-in ad-blocker, but most users use an add-on ad-blocker because they work better. Sometimes, the “disable AdBlock” is because of a false positive. Make sure you are not blocking content. Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

You can disable this feature in “Options/Preferences -> Privacy & Security” or via the about:privatebrowsing page that get when you open a New Private Window.

Options/Preferences -> Privacy & Security: "Use Tracking Protection in Private Windows"

You can check these preferences on the about:config page.

privacy.trackingprotection.enabled privacy.trackingprotection.pbmode.enabled

You can open the about:config page via the location/address bar.
You can accept the warning and click “I accept the risk!” to continue.