Firefox 3.5 came out a few days ago, and I always want to have the most updated version of everything. It won’t be in the Ubuntu repositories until 9.10 is released.

First step is from Lifehacker, it downloads and unpacks the file from the firefox website.

Open a terminal window and enter:
wget -O - | tar xj -C ~

Now, this shouldn’t overwrite firefox 3.0, but we need to move it to the mozilla folder, so use the following command:

mv ~/firefox/ ~/.mozilla/firefox3.5/

Now, make a symbolic link to firefox 3.5 in the equivalent of a “programs” folder (/usr/bin in Ubuntu):
sudo ln -s ~/.mozilla/firefox3.5/firefox /usr/bin/firefox-3.5

NOTE: you may only have one version of firefox open at one time. To open Firefox 3.5, you must completely close all other versions of firefox.

Now to set the firefox icon to the new firefox program (when opening in gnome-do and if you make a panel shortcut) open your home folder, hit ctrl-h (to show hidden files), go to .mozilla/firefox-3.5, right-click on the “firefox” in there, go to “properties” then left-click on the icon (top left corner). Now go to Pixmaps on the left (or /usr/share/pixmaps) and select firefox-3.0.png.

Now, there are a few ways to set up a shortcut to open firefox. One of the easiest is to add it to the program menu. Open the menu and go to System > Preferences > (Display >) Main Menu. Now open the “Internet” section (in the left column), and create a “New Item” with name: Firefox 3.5, command: firefox-3.5, and once again, the icon should be /usr/share/pixmaps/firefox-3.0.png.

Another method is, if you use gnome-do, just open it (alt-space) and when you start typing “firefox,” hit the down arrow and select “Firefox 3.5” (I think this only shows up after a restart, or if you add the firefox 3.5 menu item above).

Firefox 3.5 has some interesting quirks though. Flash will not work for it unless you copy your plugins into it.

In Ubuntu though, you need to copy plugins for multiple locations. Use this set of commands to copy all the plugins:
cp /usr/lib/flashplugin-installer/ ~/.mozilla/firefox3.5/plugins/ && cp /usr/lib/mozilla/plugins/* ~/.mozilla/firefox3.5/plugins/

you also need to disable the strange middle-mouse behavior in 3.5. go to about:config in firefox, click the “I’ll be careful I promise!”, and go to: middlemouse.contentLoadURL and if it is true, double-click on it to make it false.

the only thing I have yet to figure out is why all the fonts in firefox 3.5 seem to be blurry, while 3.0 was perfect… To me it is like the difference between subpixel smoothing and high contrast font rendering modes… see the screenshots below. (left is non-blurry firefox 3.0, right is blurry firefox 3.5)

UPDATE: There were a few forum topics about this issue, but nobody had a definitive solution. Many possible solutions, but many of them were kind of strange, and many did not work.

Here is what worked for me:

open a terminal, and this first step is a backup just in case the next step messes up your system’s fonts.

sudo cp /etc/fonts/conf.d/10-hinting-slight.conf /home/$USERNAME/Desktop/
sudo cp /etc/fonts/conf.d/10-no-sub-pixel.conf /home/$USERNAME/Desktop/

Now, the files were just copied to your desktop, here are the next two commands:
sudo ln -s /etc/fonts/conf.available/10-hinting-medium.conf /etc/fonts/conf.d/.
sudo ln -s /etc/fonts/conf.available/10-sub-pixel-rgb.conf /etc/fonts/conf.d/.

And finally, you need to reset fontconfig:
sudo dpkg-reconfigure fontconfig

Once this finishes, restart Firefox to see the new fixed font rendering engine! You should also restart your computer to ensure that removing those files did not do any harm. If you restart and you can still see text, then you are okay to:
sudo rm /home/$USERNAME/Desktop/10-hinting-slight.conf /home/$USERNAME/Desktop/10-no-sub-pixel.conf

Now you are finished!