Middlesex County does not prorate personal property. You will be responsible for the tax bill for the entire year.All owners of taxable personal property are required, annually, to file a Personal Property Declaration Form M-15. Pay Personal Property Taxes Online enables online payments for personal property tax bills received from Middlesex County. A single personal property assessment is made for all taxable personal property of the owner located in the city or town. 2. Search tax forms and instructions. The Tax Board's primary responsibility is the certification of property tax assessments. Other functions of the Tax Board are the equalization of assessments. Property Tax Forms and Guides. Taxpayers should use these forms and guides to apply for local tax abatements and exemptions and file property returns.