On your Ubuntu Desktop, click on System->Administration->Synaptic
In Synaptic click on settings->Repositories,and in that make sure all are ticked as shown below, especially the one which is highlighted
*img66.imageshack.us/img66/3870/ubuntulib1xm5.th.png
Click close and click the Reload button in synaptic
After everything is refreshed, click Search and look for liberation, you will get a package named ttf-liberation, mark that for installation by clicking on it and click the Apply button. Liberation fonts will get downloaded and installed on to your system.
Now close synaptic, and click on System->Preferences->Appearance
*img148.imageshack.us/img148/266/ubuntulib2vg0.th.png
In the dialog box that opens click on the fonts tab
*img356.imageshack.us/img356/95/ubuntulib3mg5.th.png *img236.imageshack.us/img236/2668/ubuntulib4rh0.th.png
Now in that you will see fonts for various purposes, application, desktop etc. Change each one to the liberation equivalent by clicking on it. So change sans to liberation sans, mono to liberation mono, sans bold to liberation sans, and in that select bold. Then close the dialog box and then open Firefox and see the difference