Ads in Firefox vs Google Chrome

I have two laptops, one Windows 7 with Google Chrome and the DuckDuckGo extension. The other machine is Linux Mint with Firefox and also the DuckDuckGo extension. A few websites I visit quite regularly, and for a while I've thought that Firefox was showing more ads than Google Chrome. So yesterday I booted both up, sat them side by side, and did some direct comparisons, just to check it wasn't my imagination.

Now confirmed, definitely more ads showing in Firefox. So bearing in mind I'm not using any addi...