To check whether it is installed as a Firefox extension, You can view your list on the troubleshooting information page. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

Then scroll down to Extensions and just below that heading, check whether you find anything from Nielsen or generally suspicious. If you'd like a second opinion, select and copy the table, then paste that into a reply. It will look a bit messy, but we're used to it. Hopefully we can spot the culprit.

If you do see an extension that needs to go, you can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right side. All extensions are optional. If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

Could you also check whether it is installed as a Safari extension?

By the way, if it's not in Firefox, it's probably best to try Apple's forums for suggestions. I see it has come up before and as a Windows user, the responses are over my head: https://www.google.com/search?q=uninstall+Nielsen+NetSight+site:discussions.apple.com