Daniel Edwards, 22, stabbed Fiona Southwell to death at Grange Farm near Seaton in July.
The jury at Hull Crown Court heard Ms Southwell, 60, replaced him as a stable hand after he was sacked.
Edwards' resentment about losing his job "may have provided him with a motive", said prosecutor David Gordon.
Edwards was sentenced to life with a minimum of 24 years.
The judge said the term would have been longer but his learning difficulties had been taken into account.
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The trial heard how Edwards looked after horses at the farm in spring 2016, but was dismissed "in a gentle and understanding way" in May 2016 and replaced by Miss Southwell.
Sentencing him, Judge Jeremy Richardson QC said: "It is plain to me that you were so angry and resentful that you hatched a plan to kill Fiona Southwell.
"You planned to kill her by stabbing her to death - you executed that plan with ferocious determination and brutality."
"The use of some of the language of the Facebook posting you later shared - you let out the demons within you," he added.
The court heard Ms Southwell's body was found in a barn at Grange Farm by her brother.
Mr Gordon said she had died from multiple stab wounds in "a vicious, protracted attack".
Edwards was arrested at his home in King Street, Hornsea, East Yorkshire. Clothing stained with Ms Southwell's blood was recovered from his bedroom.
Her blood was also found on the defendant's ankle and on a kitchen knife found in a hedgerow near the farm, the jury was told.
The court was told Edwards sought to cover up what he had done, discarding the murder weapon and lying first in his police interviews and then during his trial.
Senior Investigating Officer Det Ch Insp Tony Cockerill described the attack as cowardly, adding Fiona Southwell "never stood a chance".
Ms Southwell's family described her as "kind, hard-working and totally honest".
A man has been jailed for life after being convicted of murdering a woman who replaced him as a stable hand on an East Yorkshire farm.