In the Firefox web browser it’s possible to just open multiple tabs and access multiple pages simultaneously. if you are trying to access more than 2 pages from the same site simultaneously then those after the first two requests will hang waiting for the other requests to complete. Most people will never see this as it is rare that you want multiple pages from the same site, where there is a noticeable delay (over a few seconds). One time you may find this an issue is as a wordpress blogger. My wordpress setup is configured to notify certain search engines whenever a post is updated. This process takes some time, and if I am editing a page as well as posting a new one, or trying to upload or download large files I find that any other request ends up waiting for the earlier ones to complete.
Fortunately this is easily resolved, if you know where to look. It’s one of the settings in the firefox settings called network.http.max-persistent-connections-per-server. This cannot be reached through the normal preferences dialog, but is set by entering about:config into the URL field, and then finding the appropriate entry. In this case applying a filter (or using the find function) on the word persistent only gives two hits, one of which is the one you want to change. After changing network.http.max-persistent-connections-per-server, to 10 I can now edit multiple documents at the same time as viewing the previews / real values in another tab.