Segoe UI  Sugerido por iveyj 

AFAIK it use Win Font. Could be Segoe UI. (check Verdana and Tahoma also)

no
they are not

Default Firefox fonts:

Proportional: Serif
Serif: Times New Roman
Sans-Serif: Arial
Monospace: Courier New

SashiX i think you're talking about the fonts used by default inside webpages, not for the UI.

no
this not
this is the font of content (not the menu of browser in photo)

Man, just put pic of that font, not the whole window.

It's Segoe UI, like SashiX said. The Firefox browser uses the same default typeface as the Windows Vista Theme.