If You Installed Ubuntu from repo you would find sth missing.
for me, if i login to facebook i lost chat feature (pop up chat)
so i seek for help and found this...
(tested on kubuntu 9. 04 with KDE 4.3 RC1)

Here’s a really quick way to get 3.5 running on your Kubuntu, Ubuntu, Xubuntu 9.04 system.
Open up Terminal (Applications > Accessories > Terminal) and run the following commands:

1.cd /tmp
2.wget "http://download.mozilla.org/?product=firefox-3.5&os=linux&lang=en-US"
Note: Your download link may be different depending on your country and language. I got the link by clicking the download link, canceling the automatic download, right-clicking the “Your download should automatically begin in a few seconds, but if not, click here” link, and selecting Copy Link Location.
3.tar xvf firefox-*.bz2
4.sudo cp -r firefox /usr/lib/firefox-3.5
5.sudo mv /usr/bin/firefox /usr/bin/firefox.old
6.sudo ln -s /usr/lib/firefox-3.5/firefox /usr/bin/firefox-3.5
7.sudo ln -s /usr/bin/firefox-3.5 /usr/bin/firefox
8.Close Firefox and then reopen. You should now be running Firefox 3.5.

If for whatever reason you’d like to switch back to your previous version of Firefox, simply run the following commands from Terminal:
1.sudo mv /usr/bin/firefox /usr/bin/firefox.bak
2.sudo mv /usr/bin/firefox.old /usr/bin/firefox

Notice how I even create a backup of the original firefox before replacing it. It is always a good idea to do this if you replace programs yourself. This way you won’t be likely to remove a binary or script that you won’t be able to get back easily.
Now that you are on Firefox 3.5, check out some awesome sites that show off some of the new capabilities.
3-D cubes built with standard HTML content, including the new native video component.
Audio player built entirely with HTML, CSS, and Javascript. It doesn’t even use what we commonly think of as images, simply the new Canvas element.
Video Washing Machine – Add a video to your site, crop it to fit in a circle, put a border around it, have rollover effects that change the filter on the video, and spin the whole thing as it plays. All of this with just the <video> element, CSS 3, SVG, and a bit of JavaScript.
Add upload progress bars without the use of Flash or insane amounts of scripting.
Create the illusion of a 3D voxel engine with an image and some creative SVG, scripting, and CSS.
Apply textures to an animated model.
Create a text shadow spotlight effect using Javascript and CSS.
Web developers can now take advantage of custom typography using the new CSS rules.
Sync page elements with the position in the video. Note that the graphs are canvas elements that are being drawn on.
Use HTML elements to jump to different positions in audio.