A user pointed out - https://support.mozilla.org/en-US/questions/919567 that when you try to view an alternate image by clicking it's thumbnail on instructables.com nothing happens. This apparently worked in previous versions. I can confirm that it works in IE9. You can test this by going to http://www.instructables.com/id/Build-your-own-Electric-Car/ Below the first image of a car there are three thumbnails. Try to click on the second or third to view those images and nothing happens.
This is wfm with FF10.0.1 on win7/32 but I can reproduce the issue with SM trunk. I currently can't restart SM without Addons (adblock..) and I suspect that this is caused by an addon. Can you please try it again in the Firefox safemode ? - http://support.mozilla.com/en-US/kb/Safe+Mode
You are right - this appears to be caused by AdBlock Plus.
marking this report invalid (extension bug) It could be a real adblock+ issue if it doesn't work without a filterlist. In that case we should notify the Adblock+ developer.