I love Firefox add-ons. Love em, perhaps to a fault*. Here are a few of my favorites that I've used on load and performance testing projects.
* Ironically (for a discussion about using add-ons to enhance performance testing), add-ons can degrade Firefox's own performance. Having many add-ons active can turn Firefox into a remorseless memory-sucking monster. I've tried using a number of workarounds for keeping Firefox memory usage under control, but the best solution is to disable or uninstall as many add-ons as possible.