Abacus is an extension for Mozilla-based applications for writing and editing MathML expressions. These expressions define the presentation and the content of a mathematical expression for other user-agents on the WWW.

Abacus takes as its inspiration the W3C Amaya editor, using JavaScript and XUL to create a flexible interface for editing MathML. Abacus also insists on exporting content MathML, to convey to other user-agents the exact mathematical definition of an expression.