If you ever think that your Firefox is getting slower and slower, you might need to see Firefox's effort to improve performance by measuring add-ons on their gallery. They have published the list of add-ons which adds more load to Firefox's load time.
Quite shocking, Firebug adds 75% of the load time in Firefox 4, so your system will get slower when you enable FireBug. As you probably know, FireBug is one of the best add-on for Firefor for web developers besides Web Developer developed by Chris Pederick. But in the other hand, it also makes your Firefox becomes slower since it tried to load so many things in the background for debugging purposes.
If you are not a developer, then i guess you don't have to install this, even though you will miss a lot of goodies inside FireBug. Everything always has a trade-offs.