The problem ended up being related to the settings for my proxy server. I was able to fix the problem on my machine by adding "localhost" to the list of hosts for which Firefox bypasses the proxy server.
In Firefox 2.0, you can do this by going to Tools | Options | Advanced | Network tab and clicking the "Settings..." button in the "Connection" section on the dialog. Then, in the Connection Settings dialog, in the "No Proxy For" field, type localhost. (The new value should be separated from other host names that are already present in the field, if there are any, by a comma.)
Note that this solution is most likely only applicable if you are using a proxy server to connect to the Internet (i.e. if the "Manual proxy configuration" radio button on the Connection Settings dialog is checked).