Firestone Tire & Rubber Co. said yesterday that it earned $18 million (31 cents a share) in the fiscal 1981 third quarter compared with a loss of $32 million (56 cents) during the same quarter last year.

Sales for the period ended July 31 were $1.08 billion, slightly below the $1.12 billion of the fiscal 1980 third quarter.

The nation's second-largest tiremaker attributed the results to better efficiency in its North American tire operations, continued strength in sales of replacement tires and substantial reductions in interest expense.

For the first nine months of the fiscal year, Firestone said it earned $121 million ($2.09) on sales of $3.2 billion. In the comparable period last year, the company lost $98 million on sales of $3.5 billion.

This year's nine-month net income includes $50 million from the sale of its plastics company and $6 million from an extraordinary tax credit. The 1980 nine-month loss included a $49 million after-tax charge for the closing of seven plants in the United States and Canada.

Deere & Co. said its fiscal third-quarter profit more than doubled from the same quarter last year, mainly because of foreign exchange gains and improved demand for its products compared with a year earlier.

The manufacturer of farm and industrial equipment said it earned $67.1 million (99 cents a share) in the quarter ended July 31 compared with profit of $27.6 million (45 cents) a year earlier. Sales rose by 8 percent to $1.47 billion from $1.37 billion.

In the first nine months of its fiscal year, Deere earned $195.7 million ($2.97) on sales of $4.01 billion. In that period a year earlier, it earned $183.2 million ($3) on sales of $4.02 billion.

Westvaco Corp., a paper and chemical combine, earned $25.39 million ($1 a share) in its third quarter ended July 31, up from $20.09 million (79 cents) a year ago, as sales rose to $392.69 million from $349.04 million.

Nine-month net income was $70 million ($2.75) on sales of $1.149 billion compared with $60.16 million ($2.37) a year earlier on sales of $1.036 billion.

Chairman David Luke III said good gains for three successive quarters reflect the success of the company's restructuring.