I've tried several times now and haven't been able to reproduce the problem.
If it only happens at times and not entirely reproducible, my guess is a memory problem or something related. Even the latest versions of Firefox are prone to consuming high amounts of memory in prolonged use, and stable Firefox builds for Windows are only available in 32 bit (which, under normal circumstances, can only use up to 2GB of memory). The function you're talking about is an AJAX script that may cause a supposedly slight increase in browser memory use, which may have caused or accelerated the freezes.
Another possibility is the presence of a browser security toolkit, often the byproduct of an anti-virus programme such as Avast and AVG. Some such addons may attempt to filter or intercept resources like website scripts and cross-check them with the anti-virus vendor's database to help ensure that your browser load no malicious component. A choke in the process may cause your browser to lag. As some like to put it, "shit happens."