If you are using Firefox, chances are you use more than a few Add-ons or extensions. After all, they have been one of the main strengths of the Mozilla browser. If you do, though, you should know that such extensions might have an impact on performance. In fact, the impact can be so significant that Mozilla themselves have decided to publish a list with the 10 (on the main view, there are more listed) Add-ons that slow Firefox down the most and surprisingly, that list includes some of the most popular extensions available!
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I personally only use a couple extensions myself, none of which is listed here, but if you do use many and feel Firefox takes forever to start up or has less than acceptable performance, you may benefit from visiting this SITE and learning which extensions could be the root cause of your problem. Most popular extensions have alternatives available, so you might be better off disabling that particularly slow Add-on and installing a similar one.
If, on the other hand, you are an Add-on developer, please make sure you are clear on Mozilla's Performance Best Practices in Extensions.