Adding a rule to ABP blocking ‘http://www.lijit.com/*’ seems to have cured the problem though: returning to the Lijit-infected sites hasn’t hit me with any popups so far. At some point, I should investigate further – the popup shouldn’t be getting through Firefox’s own popup blocker, even without ABP.

It looks as if the Lijit code on those sites tries to load a copy from the Lijit servers, then falls back to using an embedded copy if that fails – ingenuity and fault-tolerance I’d applaud, were it used for good rather than evil. If it’s that resilient and the popups persist, I suppose I’ll have to go and put a ‘placebo’ version of their Javascript on another server and point there with /etc/hosts. Very irritating, and not at all what I would expect from any “search engine”!