mouth off vs jabber

mouth off is not a noun while jabber is a noun.

Word	Noun	Adjective	Verb	Adverb
mouth off	No	No	Yes	No
jabber	Yes	No	Yes	No

As verbs, mouth off and jabber are synonyms defined as:

Other synonyms of mouth off include rabbit on, rant, rave, spout.

mouth off (noun)	jabber (noun)
	rapid and indistinct speech

mouth off (verb)	jabber (verb)
talk in a noisy, excited, or declamatory manner	talk in a noisy, excited, or declamatory manner