The term firebrand is a noun which means to be an agitator,
demonstrator, or rebel. An antonym for the word firebrand is
peacemaker.
View page
The mostly UK slang "tubthumper" is probably equivalent to
"rabble-rouser."
The most kind word for a vigorous promoter would be activist,
although other synonyms could include dissident, radical,
firebrand, fomentor, instigator, or demagogue.