Some firewalls like Kaspersky can block content and send a 1x1 GIF image instead. Kaspersky": "Protection > Firewall > Filtration system: disable the "Banner Ad Blocker"

See Configuring Kaspersky Internet Security and Configure firewalls so that Firefox can access the Internet

Why would I want to do that? I want to keep adds blocked. Come on - build your browser to get around this - I'll use another browser first.

It is Kaspersky that is doing that. It intercepts the content and passes that image to Firefox instead of the real content. You should complain on the Kaspersky forum about this if you disagree with the decision made by Kaspersky to send such an image in this case.

You can look at Adblock Plus instead.

• Adblock Plus: https://addons.mozilla.org/firefox/addon/adblock-plus/

You need to subscribe to a Filter list (e.g. the EasyList).

• http://adblockplus.org/en/subscriptions
• http://adblockplus.org/en/getting_started

ok I'll take a look - I assume the same problem with other browsers too then?

Not if they only do that for Firefox or have a browser specific setting.
We regularly see this issue reported on the Support forum.

• https://support.mozilla.com/en-US/search?q=Kaspersky%20banner&sortby=1&w=2&a=1