Adblock is not a panacea. It doesn’t block all ads, and it doesn’t block malicious scripting in the form of non-ads. It does a great job in blocking most ads, but Adblock relies on two things to work properly: the first, a subscription list which is updated as new ad sources are discovered; the second, manual intervention by the end-user to block ads the subscription list does not cover. Because of the current model, a small number of ads will always peek through.
Its usefulness is also limited by an end-user being tricked into willingly downloading and installing a “video codec” or other trojan horse. You’ll see this a lot on Facebook or other social media sites where a friend’s hijacked account asks you to click on a link to view a “funny video”. Either the linked-to site has malicious code that runs through Firefox’s (or, insert browser) security holes, or it tricks the user into downloading and installing a malicious file.
Right now I’m not convinced you were infected by malware on this site. I say that for numerous reasons, but mainly because, at this point, you’re the only one making a complaint. In the past when the SDMB has been the victim of a malicious ad, numerous people get hit. It’s not just one. It’s not impossible that you’re the only one to get infected, but with the high traffic of this site, it’s not very likely.