Firefox is slow in Linux compared to XP or even Vista on the same hardware. Do a quick google search for "firefox slow linux" and you see it's not just me who's saying this.
I tried a couple of tips from other users in discussion forums such as disabling visual effects and IPv6 but it didn't really help. That's why I started thinking about installing Firefox 3.5 on my Ubuntu 9.04 64-bits machine to see if it makes a difference.
At first I though I will have to download it directly from the Firefox website, however to my surprise I found the package in Synaptic. I am not sure which repository it came from but I can see the package "firefox-3.5" from the Synaptic package manager.
I went through the usual installation process and frankly it did not prompt me to remove existing firefox-3.0 packages. Instead, it installs the Firefox 3.5 alone side Firefox 3.0 under the name Shiretoko Web Browser in the Application -> Internet -> Shiretoko Web Browser.
Could be just my wishful thinking but Firefox 3.5 does seems to be a bit faster than 3.0.