Answer :
The statement "his mouth full of brimstone" begin emphasis,most likely,end emphasis, meant "The man spoke in a fiery and vulgar manner".
Meaning of fiery and vulgar
Fiery and vulgar are both adjectives meaning glowing, inflammable or being ignited and inappropriate, offensive, uncouth, mean respectively.
This means the bearded, mildewed little man spoke in an offensive and inappropriate manner. He spoke with so much fire in his eyes.
Therefore, the statement meant ""The man spoke in a fiery and vulgar manner".
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