Most online virus scanners require use of the Internet Explorer web browser to work. For those of us who use Firefox as our default browser and sneer in disgust at the ineptitude or laziness of the particular site developers there is a way to circumvent this issue.
Simply install the IEtab addon via IEtab installation page
Once the addon is installed simply navigate to the 'Tools' menu > Add-ons > IE Tab options.
Insert the URL in the site filter. You may have to use regular expressions or wildcard characters.
For example: http://www.windowsupdate.com/*
By default Microsoft update is included in the site filter so that users do not have to jump into IE if they automatically update.
BTW, since essentially these sites are displayed in Firefox they still utilize the IE engine. Don't uninstall IE.