Du Pont Co. reported a 19 percent gain in earnings for all of 1979 but a downturn in final quarter profit caused largely by increased costs of petrochemical feedstocks and higher silver prices.

Profit for the year was $939 million ($6.42 a share), up from $787 million ($5.39) in 1978.

Sales were $12.572 billion, up from $10.584 billion the previous year, another gain of 19 percent.

Final quarter profit slipped 7 percent from a year ago to $206.1 million ($1.41) on revenues of $3.209 billion from $220.1 million ($1.51) a year earlier on revenues of $2.718 billion.

Norton Simon Inc. earned $1.41 a share in its first half ended Dec. 31, up from $1.29 a year earlier, but chairman David Mahoney said the results were adversely affected by many factors.

Second quarter profit was 75 cents a share, up from 70 cents last year.

Mahoney listed higher interest charges, falling demand for the Avis subsidiary's rental cars, some unfavorable foreign currency translations, lower profit margins in the food and soft drink divisions and high raw material costs in the packaging business. The Somerset liquor imports; McCall Patterns and Max Factor & Co. did well, he said.

Net income for the half was $69.19 million on earnings of $1.496 billion against $53.6 million a year earlier on revenues of $1.393 billion. For the second quarter, net income was $36.61 million on revenues of $765.63 million compared with $34.29 million a year earlier on revenues of $696.63 million.

Pfizer Inc., the chemical and pharmaceutical house, reported a 13 percent gain in earnings for last year to $3.26 a share from $2.89 in 1978.

Sales grew 14 percent to $2.746 billion from $2.399 billion and net income rose to $237.9 million from $209.9 million.

However, the fourth quarter earnings gain was only 4 percent to 80 cents a share from 77 cents a year earlier in spite of a continuance of the 14 percent sales rise to $743.8 million from $650.7 million. Net income advanced to $58.6 million from $56.1 million. The net income rise was 7 percent; the per share gain was less because more shares were outstanding in 1979.

Declining sales of glass and plastics to the automobile industry slashed earnings of Libbey-Owens-Ford Co. sharply for both the final quarter and all of 1979.

Profit for the year was $58.08 million ($4.80) on the sales of $1.208 billion compared with $65.1 million ($5.50) in 1978 on sales of $1.107 billion. b

Final profit was $9.03 million (70 cents a share) on sales of $296.88 million compared with $15.82 million ($1.32) a year earlier on sales of $286.45 million.

The 1979 earnings were reduced 37 cents a share by changing all the company's inventory accounting operations whereas 1978 profit was increased 54 cents by the net effect of a gain on sales of securities after an offsetting loss of 53 cents a share on a plant closing.

McDonnell Douglas Corp. reported 1979 earnings totaled $199.1 million ($5.06), an increase of 23.6 percent from 1978. Sales climbed 27.8 percent to $5.3 billion last year from $4.1 billion the previous year.

The St. Louis-based aerospace company had a profit of $161.1 million ($4.14) on 1978 sales of $4.1 billion. Its commercial jet aircraft include the DC10 wide-body plane.

During the fourth quarter, the company said it earned $51.7 million on sales of $1.33 billion, compared with $43.9 million in profits on sales of $1.17 billion for the final three months of 1978.