This is happening to me on most forums I've visited and in most threads I'm in. When the page loads, the McAfee security banner displays at the top of the screen, and whatever ad was displayed at the bottom of the page to the right is greyed out with a round STOP sign in the corner of it. If you reload the page, or click Back, the ad will for a split second re-appear. Mostly it seems to be an ad for a phone or something, it's quite hard to tell.
If I click back, it doesn't work, it just reloads the same page, and I have to go into the Back menu and select the page I want to go to. very bloody irritating.
not sure if this is quite the right place for this, but couldn't think of anywhere better.