Andavari, thanks for your suggestion. I've downloaded the latest version and no longer have this problem. It seems I had updated on the 5th April, and the current version came out on the 9th. duchess, you're welcome!
Earlier this month (having not updated CCleaner Free for a long time), I was informed I needed to update. So I downloaded ccsetup555_rtb.exe and ran it. CCleaner now says I'm running CCleaner Free v5.55.7109 (64-bit). However, whenever I log into Windows, Firefox (my default browser) pops up with the CCleaner download page (). This is despite the fact I didn't last have that page open in Firefox (the browser remembers your last page). How can I stop this?