The initial WGA 1.0 program downloaded an ActiveX control to check the authenticity of your Windows software. Since it was an ActiveX control, only Internet Explorer (IE) users could use it.

But the recent growth of Firefox has forced Microsoft to rethink their strategy. Microsoft Genuine Windows Validation process now works in Firefox and other Mozilla browsers. Firefox users can download and install the Windows Genuine Advantage validation Firefox plug-in - WGAPluginInstall.exe available on Microsoft’s website to complete the Windows validation process.

Download: WGA Plugin for Firefox
Source: MSFN