The European Commission has granted Mozilla the right to join its anti-trust case against Microsoft.
The Commission, Europe's top antitrust authority, charged Microsoft last month with distorting competition in the market for web browsers by bundling IE with the Windows operating system.
If the charges stick, then Microsoft could be forced to change the way it distributes IE, as well as pay a fine for monopoly abuse.
In a blog, Mitchell Baker, Mozilla's chairperson said that she wanted to offer Mozilla's expertise "as a resource to the EC as it considers what an effective remedy would entail."
She said there isn't "the single smallest iota of doubt" that Microsoft's tying of IE to Windows "harms competition between web browsers, undermines product innovation and ultimately reduces consumer choice."
Mozilla has been granted what's called "interested third party" status in the case, which allows it to submit arguments to the European regulator, to see the confidential statement of objections the EC sent Microsoft last month, and to participate in a face-to-face hearing if Microsoft requests one.
However, it isn't a complainant in the case. That role goes to Norwegian web browser Opera, which complained to the EC just over a year ago about Microsoft's practices in the browser market.
Firefox is IE's nearest rival in Europe, according to market share data from French researcher XiTiMonitor.
Last November IE's usage share in Europe stood at 59.5 percent, Firefox 31.1 percent, Opera 5.1 percent, Safari (Apple's browser) 2.5 percent, and Google's recently launched Chrome browser at 1.1 percent, XiTiMonitor said.
Google and Apple weren't immediately available to comment on whether they too would apply to join the EC's anti-trust case.