(1) 'their' refers to 'men'
(2) 'others' refers to other 'men'
'men, who do not much think what they are saying, bestow [praise] sometimes upon others, and sometimes on themselves; and their imaginary dignity is naturally heightened by a supercilious censure of the low, the barren, the grovelling, the servile' -- this whole section refers to the nature of men in general, and is only secondarily applied to the imitative vs. original artists in the first paragraph, NH.
(3) 'praise' means expressed admiration or commendation-- in this case upon those who are original artists instead of imitators.
Student or Learner