Firefox is one of the top most popular and commonly used web browser because has features like Addon..etc.Even Firefox has lot of best addons also If you find your firefox is performing slow then it may be because of slow performing addons installed in your browser.Add on developers already listed some of the slow performing addon have significant impact on how long it takes Firefox to start up.
Check If any of the above addons already installed in your browser and disable(How to disable) immediately to achieve better firefox experience.From the above Screen Firebug addon may cause more impact in firefox performance.