No, Segoe UI does not come bundled with Firefox.

Segoe UI is the default for interface elements for Windows starting with Windows Vista and Microsoft Office 2007. So, Segoe UI and other Segoe variants can be obtained by installing Office 2007 or later, or Windows Vista or later.

More information can be found here:

• https://www.microsoft.com/typography/fonts/family.aspx?FID=331
• http://en.wikipedia.org/wiki/Segoe#Segoe_UI

Therefore, machines running Windows XP, or non-windows OSes might not have this font installed. Looking at the Metro bootstrap CSS it should fall back to "Helvetica Neue". Which is a font that is installed on most Mac's.

If none of Segoe UI variants or Helvetia Neue it falls back to the default Sans-serif font in your browser. Ironically, this seems to be Segoe UI (at least, on my machine).

Since this too doesn't render the page in Segoe, i assume that the system where you are running chrome on, doesn't contain the Segoe fonts.

As far as I am aware, Word for Mac does not install the Segoe fonts.

Its difficult to get this working in Firefox. Font weight 300 doesnt work few time in all versions. The below code worked for me and compatible with all browsers.

 font-family: "Segoe UI Light","Segoe UI";
 font-weight: 300;

See Here This is from a HTML5 solution, but it might help you too, as it's also in Visual Studio... Hovering over the CSS font-weight options will tell you the weight in words (Light, Semi, etc.) 100: Thin 200: Extra Light (Ultra Light) 300: Light 400: Normal 500: Medium 600: Semi Bold (Demi Bold) 700: Bold 800: Extra Bold Hope it helps.

Follow the below options and add font-weight instead of using semibold or semilight.Just use 'segoe ui' with combination of font-weight.

    @font-face {
    font-family: "Segoe UI";
    font-weight: 200;
    src: local("Segoe UI Light");
}
@font-face {
    font-family: "Segoe UI";
    font-weight: 300;
    src: local("Segoe UI Semilight");
}
@font-face {
    font-family: "Segoe UI";
    font-weight: 400;
    src: local("Segoe UI");
}
@font-face {
    font-family: "Segoe UI";
    font-weight: 600;
    src: local("Segoe UI Semibold");
}
@font-face {
    font-family: "Segoe UI";
    font-weight: 700;
    src: local("Segoe UI Bold");
}
@font-face {
    font-family: "Segoe UI";
    font-style: italic;
    font-weight: 400;
    src: local("Segoe UI Italic");
}
@font-face {
    font-family: "Segoe UI";
    font-style: italic;
    font-weight: 700;
    src: local("Segoe UI Bold Italic");
}

As the other answers note, Segoe UI is not available for use in web apps not made by Microsoft or their agents. To answer the title of your question, however, I'd like to point out that Microsoft themselves use Segoe UI in their web apps via @font-face, eg, from the Microsoft Surface 2 CSS:

@font-face {
    font-family: 'Segoe UI';
    src: url('/surface/Assets/css/fonts/all/normal/segoeui.eot');
    src: url('/surface/Assets/css/fonts/all/normal/segoeui.eot?#iefix') format('embedded-opentype'),
         url('/surface/Assets/css/fonts/all/normal/segoeui.woff') format('woff'),
         url('/surface/Assets/css/fonts/all/normal/segoeui.svg#SegoeUI') format('svg');
    font-weight: normal;
    font-style: normal;
}

@font-face {
    font-family: 'Segoe UI Semibold';
    src: url('/surface/Assets/css/fonts/all/semibold/seguisb.eot');
    src: url('/surface/Assets/css/fonts/all/semibold/seguisb.eot?#iefix') format('embedded-opentype'),
         url('/surface/Assets/css/fonts/all/semibold/seguisb.woff') format('woff'),
         url('/surface/Assets/css/fonts/all/semibold/seguisb.svg#SegoeUISemibold') format('svg');
    font-weight: normal;
    font-style: normal;
}

@font-face {
    font-family: 'Segoe UI Bold';
    src: url('/surface/Assets/css/fonts/all/bold/segoeuib.eot');
    src: url('/surface/Assets/css/fonts/all/bold/segoeuib.eot?#iefix') format('eot'), /* Wrong format will tell IE9+ to ignore and use WOFF instead. MSHAR-2822 */
         url('/surface/Assets/css/fonts/all/bold/segoeuib.woff') format('woff'),
         url('/surface/Assets/css/fonts/all/bold/segoeuib.svg#SegoeUIBold') format('svg');
    font-weight: normal;
    font-style: normal;
}

@font-face {
    font-family: 'Segoe UI Light';
    src: url('/surface/Assets/css/fonts/all/light/segoeuil.eot');
    src: url('/surface/Assets/css/fonts/all/light/segoeuil.eot?#iefix') format('embedded-opentype'),
         url('/surface/Assets/css/fonts/all/light/segoeuil.woff') format('woff'),
         url('/surface/Assets/css/fonts/all/light/segoeuil.svg#SegoeUILight') format('svg');
    font-weight: normal;
    font-style: normal;
}

Even projects like Metro UI - that look exactly like Windows 8 - use OpenSans:

@font-face {
  font-family: "Open Sans";
  font-style: normal;
  font-weight: 700;
  src: local("Open Sans Bold"), local("OpenSans-Bold"), url(https://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzJ1r3JsPcQLi8jytr04NNhU.woff) format('woff');
}
@font-face {
  font-family: "Open Sans";
  font-style: normal;
  font-weight: 300;
  src: local("Open Sans Light"), local("OpenSans-Light"), url(https://themes.googleusercontent.com/static/fonts/opensans/v6/DXI1ORHCpsQm3Vp6mXoaTZ1r3JsPcQLi8jytr04NNhU.woff) format('woff');
}
@font-face {
  font-family: "Open Sans";
  font-style: normal;
  font-weight: 800;
  src: local("Open Sans Extrabold"), local("OpenSans-Extrabold"), url(https://themes.googleusercontent.com/static/fonts/opensans/v6/EInbV5DfGHOiMmvb1Xr-hp1r3JsPcQLi8jytr04NNhU.woff) format('woff');
}
@font-face {
  font-family: "Open Sans";
  font-style: normal;
  font-weight: 400;
  src: local("Open Sans"), local("OpenSans"), url(https://themes.googleusercontent.com/static/fonts/opensans/v6/K88pR3goAWT7BTt32Z01mz8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}

I would recommend OpenSans.