By default, Firefox uses a serif type font for HTML pages where a page does not declare a font to use. Sans serif fonts are much easier to read on the screen whereas serif fonts are better for printed material. To change the default font Firefox uses, go to Preferences (which can appear under Tools in Windows, Edit in Linux, and I believe Firefox in OS X) and select the Content tab. Change the default font to a sans serif font such as Arial and make those same changes under the Advanced button.
Thunderbird also uses serif fonts by default, including for any e-mail you may send someone. Change the default font to sans serif in the Preferences area of Thunderbird under the Display section, Formatting tab, and select the Fonts button.