Mozilla boss Harvey Anderson previously hit out at the firm following reports that its own Internet Explorer will be given special privileges for the ARM version of OS, making it the only browser able to run in 'Classic' mode.
The Commission said that it will monitor the situation to ensure that Microsoft abides by antitrust laws, but has not yet launched an official investigation.
"The commission is aware of these allegations and will remain vigilant that Microsoft fully complies with its commitments under the commission's 2009 decision on browsers. This decision applies to Microsoft's Windows operating system for PCs," said EU spokesperson Antoine Colombani.
Microsoft was forced to offer users of its operating systems a choice of web browsers following a 2009 agreement with the European Commission.
The company's Windows 8 RT platform is under similar scrutiny in North America from the US Senate's antitrust subcommittee.
Microsoft will release a preview build of Windows 8 in the first week of June.