Notice: This addon is no longer maintained. If you'd like to update it for newer Firefox versions check out the Github repo: https://github.com/djungowski/ff-dnscache
After the installation, there's a new button in the Add-On Bar of Firefox that allows you to enable and disable the DNS cache by a simple double click. Right click opens a contextmenu which allows you to also flush the DNS cache manually.
If you prefer to have a button in the navigation/main panel, that's the way to go:
Just right click on your main panel and select "Arrange", now you should see a new icon for the DNS Cache, which you can easily drag and drop to where you would like to have it