But there are few things that you can do to make it a little bit "speedier". Remember though, some of these customizations will disable flash or DRM playback, so unless you don't need them you should not disable them.
Open your Firefox4 and from the menu go to, "Tools" -> "Addons" and then you'll be greeted with a window similar to the below one...
*. Microsoft DRM - This as said before, enables Firefox to play DRM files via websites. So if you disable this then you won't be able play those.
*. Real Player version plugin/ Real Player G2 LiveConnect.... - This is related to the real player plugins which enables playing online real player files (audio/video). So only disable it if you don't need em.
*. Shockwave Flash - Well, if you have a slow internet connections or have a threshold to "defend" (you know a usage limit cap), and don't want to play videos or anything that is flash related all the time then disable it. It should help you to save some of those data-usage issues.
*. Windows Media Plug in Dynamic Link .... - This is associated with playing windows media files online. So, if you don't need that, then disable it too.
Try not to install addons that has no real need for you. Not only they add up HDD space but most importantly they slowdown Firefox. Treat the same to unnecessary tool-bars and extensions.