Firefox performance is extremely important to our users, especially how quickly it starts up and loads websites. Customization is also extremely important, and while most add-ons cause only a tiny performance impact, others can significantly slow down Firefox. Many users don’t realize add-ons can cause these delays, and that’s why we’re committed to improving performance in a big way.
On average, each add-on you install adds about 10% to Firefox start-up time. For some users that’s mere milliseconds, and for others it may be half a second; it all depends on the hardware and software of each individual. Many add-ons add less than 10%, and unfortunately, there are quite a few add-ons that add more. But based on our real-world performance data, installing 10 add-ons will double Firefox’s start-up time.
via Mozilla Add-ons Blog – Improving Add-on Performance. Nice, that’s a great thing for users, start publishing information regarding the performance of add-ons and start trying to convince programmers to be more intelligent in building add-ons. The number one slowest add-on is Firebug (which makes sense but is awful for web developers), also in the top ten is an add-on supposedly designed to speed up Firefox.