I test with fresh user profiles (default installations, no add-ons) only. Basic Adblock is disabled by default in TFF.
That said, today it works here, too (repeatedly in FPR 11 but not Safari 5/LWK). Maybe the site is a bit flaky after all. At least I get this (see screenshot), even though I do not need to solve a captcha. Is that what it's supposed to look like (I've never used Craigslist before)?
I get the same thing in both FPR11 and debug FPR12 as well. As I recall this is the normal functioning reply button for craigslist though I haven't used it recently. Willy, do you see something different?
I didn't need to solve a captcha either, though I saw the captcha icon appear briefly at the lower right of the window while the spinning cursor appeared in the centre.