How come that Firefox on Window seems to render font correctly even if these are not installed on the system? The style sheet says ´Street Plain´ and Firefox displays it correctly but i didn´t find it among the installed fonts. I´m working with Windows 7 (64Bit) and Firefox 3.6.15

Probably it's a web font...

Web font is compatible :

Google Chrome: version 4.249.4+
Mozilla Firefox: version: 3.5+
Apple Safari: version 3.1+
Opera: version 10.5+
Microsoft Internet Explorer: version 6+

http://code.google.com/apis/webfonts/docs/getting_started.html#Quick_Start

Thanks for your answers, very appreciated