When I first started using Ubuntu, I found that the default fonts come with its Firefox are a bit difficult to read. By default, Ubuntu doesn't have popular Microsoft fonts such as 'Times New Roman' pre-installed. So the first task I did was to install Microsoft True Type core fonts bundle on Ubuntu. It can be done with a simple apt-get command as following:
sudo apt-get install msttcorefonts
This installs all the required Microsoft fonts. Then all I had to do was just to change the font settings in Firefox to be similar to Firefox on Windows.
Preferences -> Content
Default Font: Times New Roman
In Advanced Settings:
Fonts for: Western
Propotional: Serif Size: 16
Serif: Times New Roman
Monospace: Courier New Size: 13
Note that these settings are similar to Firefox on Windows 7.