I'm posting this from Crunchbang and it's fine for me.
Check the Firefox font settings: Edit>Preferences>Content>Fonts and Colours Default font: serif Size: 16
Then "Advanced" Proportional: Serif Size: 16 Serif: serif Sans-serif: sans-serif Monospace: monospace Size 12
Check the box for "Allow pages to choose their own fonts..."
Also have you installed ubuntu-restricted-extras which supplies w32codecs (or w64codecs if appropriate)? Either do this from Synaptic or the Terminal
sudo apt-get install ubuntu-restriced-extras