Judge Robert H. Bork issued an impassioned dissent yesterday from an appellate court finding that a postal worker was improperly fired for exercising free speech.

Bork, President Reagan's embattled nominee for the Supreme Court, said his colleagues on the U.S. Circuit Court of Appeals here had "created new First Amendment law that runs directly contrary to Supreme Court precedent."

The case involved a U.S. Postal Service clerk who was fired in 1983 after he wrote a column in his union's newsletter based on a piece of third-class mail he said he had taken from the Royal Oak, Mich., Post Office where he worked. The letter was part of a mass mailing by Rep. Philip M. Crane (R-Ill.) that solicited support for a bill to limit union organizing tactics.

The appeals court ruled, 2 to 1, that Joseph V. Gordon should be reinstated in his job because the Postal Service had punished him for exercising his First Amendment right of free speech. It upheld a decision by U.S. District Court Judge Gerhard Gesell that the firing was "substantially motivated by Gordon's protected speech."

On facts stipulated by both sides, Gesell ordered Gordon's reinstatement but said the Postal Service could conduct administrative proceedings to impose a less severe punishment. The appeals court reversed that finding, saying that no punishment was warranted because Gordon's article was constitutionally protected speech.

The Postal Service's statement that it fired Gordon for disclosing the contents of mail "was merely a pretext" for punishing him for writing the column, the majority said.

Bork said Gordon's dismissal was too harsh, but he added, "The severity of the punishment, however, is not before us and we have no power to alter it.

"Gordon's speech directly threatens public confidence in the privacy of the mail stream," Bork said. "It encourages other postal workers to invade the privacy of the public mails in order to dramatize whatever personal statements they wish to make."

First Amendment rights will be curtailed in general "if members of the public come to believe that they may not exchange their thoughts and opinions through the mails without fear of disclosure."

"As far as the First Amendment is concerned, today's decision is in my view a truly Pyrrhic victory," he said.