For firefox exists a huge bunch of plugins. It’s like with eclipse. The more people it use more people write extensions to it. Here are a list of plugins which i use:
to see bugs in dhtml…
to integrate other downloadmanager like fdm
to see local weather in the status bar
to use ie inside firefox
to have links in text where html link isn’t set (links in plaintext)
to handle pdf viewing
to handle source code view
to analyse web pages