In "I felt a Funeral, in my Brain," Emily Dickinson uses the extended metaphor of a funeral service to describe her thoughts. The tone is bleak and desolate, as one might expect a funeral to be. The repetition of the words "treading" in the third line and "beating" in the seventh reinforce a sense of monotony that becomes a form of torture, breaking down the poet's defenses and numbing her mind. In the third stanza the "Boots of Lead," ominously capitalized, add to the image of heavy footsteps treading across her psyche and leaving an unwelcome footprint.
This somber mood* continues throughout the poem. However, it is increasingly accompanied by a sense of mysticism and even surreality. The images become stranger and more difficult to follow, with Heaven described as a bell, and Being as an ear. In the final stanza the sense of alienation is heightened by insistent polysyndeton, creating the impression of one thing following another with nothing the poet can do to arrest her sudden descent:
And then a Plank in Reason, broke,
And I dropped down, and down -
And hit a World, at every plunge,
And Finished knowing - then -
The poet seems to lose control, falling into madness and never being able to tell the reader what she learned as she fell, finishing the poem on an inconclusive, mysterious note.
*The question asks about both tone and mood. These will be the same for all practical purposes, since it is the tone that creates the mood.