Firefox still the best browser ever, at the moment after i had install firefox 4 in my laptop i felt there was a little bit change in firefox, not because their new interfece but I feel this browser walk slowly than it's previous, Is your Firefox 4 walk Slowly?, Find Out why it's happen..
Regarding to the mozilla firefox official website, Some add-ons may even slow Firefox to a crawl and make it difficult to use for regular web browsing, for best performance you should disable add-ons that you no longer use regularly. When i checked at my firefox evidently i have activated one of most influential add-ons which have most impact on how long it takes Firefox to start up. Check the list below, maybe one of them installed on your firefox 4.
Top 10 Most Slowing Performance add-ons
To get the complate list of Most Slowing Performance add-ons, you can go to this link. If you find some add-ons above installed on your pc/laptop you should consider to delete them unless that add-ons is very important to you. Besides of avoiding some add-ons above we have small tips which can improve your firefox performance. Launch your Firefox 4 and in the address field, type about:config and hit ENTER. Now it will show you a warning message, click on the button labelled as I’’l be careful, I promise. You will face with Mozila Firefox Config page, you need to change some parameter to get your firefox running fast, find some parameter below and change it according to the example below:
network.http.pipelining => true
network.http.proxy.pipelining => true
network.http.pipelining.maxrequests => 12
content.notify.backoffcount => 5
plugin.expose_full_path => true
ui.submenuDelay => 0
network.dns.disableIPv6 => true => true

javascript.options.jit.content => true
Last tips, try to use ads blocking add-on to reduce heavy workload on Firefox 4, to much ads will slow down your firefox 4 performance. There are many Firefox 4 addons available on Internet which would help you block ads. hope that this will surely increase the speed of Firefox 4.