This is from the Firefox help file:
"Overriding Page Fonts
Some web sites may display text in fonts that are difficult to read, and other sites may rely on the browser's default fonts. You can set your default fonts by going to Tools > Options, selecting the Content panel, and selecting a default font and size in the Fonts & Colours section.
You can specify default fonts for different styles, including serif, sans-serif, and monospace by clicking the Advanced button.
However, many web pages specify their own fonts, so your choices will have no effect on such pages. To override the fonts defined by such pages, untick the Allow pages to choose their own fonts, instead of my selections aboveoption.
Note that some web pages may display incorrectly depending on your choice of a default font."
To change the font associated with the icons on your desktop, right-click on the desktop. Select 'Properties' and 'Appearance'. Set 'Item' to show 'Icon' and then select your preferred font. Click 'Apply' and 'OK'.
Chris