you are correct, I am getting the error as well. I am still attempting to figure out what....I was thinking to blame turtle but eh I'm being a good boy today......
The forum does use several add-ons that could be causing it... especially if your protection software has a false positive or they are being really picky about minor spyware/tracking issues... I'm going to ignore these possible causes simply because they require installing other browser extentions/add-ons that I'm unfamiliar with since I don't use FF ( I've been using Chrome with an extention called Ghostery that blocks most of these types of things and haven't had any issue on any of my systems )... Since the issue isn't reported for the first page ( from what I've read ), I doubt it's an ECF add-on causing it since they'd be enabled/active on all pages.
So to track down other non-ecf things and only look at things caused by a users post... Go into your forum settings-> general settings.... scroll down to the thread display options... first disable ( remove the checkmarks for ) the show signatures, avatars and images... scroll to the bottom and save the changes... Go back to the thread in question and see if the issue shows up again or not. If it doesn't have the issue then... enable those options one at a time to find out which area to focus in. If the problem still exists with all of those options disabled... just re-enable them.
Assuming you found one of the three options is causing it... or you didn't... to isolate it further... in settings-> general settings-> thread display options again... select to only show 5 posts per page. This will help allow you to isolate which post ( within the 5 ) that may have the issue. By going through all 30 plus posts ( 6+ pages ) you may be able to see which user's post causes the trouble. At the very least you can use a process of elimination to say who's post may or may not be a possible cause if they posted in a page of posts on the thread and didn't have the issue show up.