Firefox's popup blocker should never block a new window when you click the link yourself. It's possible that the link has a script attached to it that handles the click in a manner other than an ordinary click. Can you post a link to a page that exhibits this problem?

Also, could it be a NoScript message rather than a Firefox message?

I don't believe it's noscript -- the box that comes up is the standard Firefox pop-up blocker.

Any image thumbnail on 4chan exhibits that behaviour for me. The links, for example, are of the following form: <a href="//images.4chan.org/b/src/######.jpg" target="_blank">######.jpg</a> Now, I'm not aware of any stylesheets applied to this which might trigger a script, but again, the only setup I have where this is causing problems is on the one described.

Note that my Fedora 17/Firefox 13.01 setup does not have this issue, despite having similar add-ons installed.

I have no problem displaying images on 4chan even with the pop-up blocker on (Windows 7). Something may be altering the pages on your Vista system.

A standard diagnostic for interference by add-ons is to try Firefox's Safe Mode.

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the Safe Mode dialog, do not check any boxes, just click "Continue in Safe Mode."

If the site works correctly, this points to one of your add-ons or custom settings as the problem.

Any change?

Excellent -- that helped me narrow down the problem to something to do with AdBlock 2.1.1 and 2.1.2 (after upgrading to see if it had been fixed). I seem to recall upgrading that add-on recently, so that's probably right. I can disable everything else, and just enable that add-on, and the problem still occurs.

Thanks for you assistance! I've attempted to contact the add-on developers on their website and consider this closed as far as the Mozilla side of things is concerned (although it might be helpful if someone else could confirm this issue).