I only have 3 Firefox extensions installed. FlashBlock is one of them.
Pros: It replaces flash animation with a button which user can click to start the flash content. Since 99% of flash contents on the web are Ads that just distract your attention from reading and slow down the CPU, this extension is a big saver. It also has a white list option to allow some customization.
Cons: Uninstall is not as easy as install. There are other known problems.