It was Dirty Frank, not Filthy Frank. He lived in an old cardboard box and was definitely styled after Oscar the Grouch. He had a screaming Boston accent, so likely voiced by someone from the production crew.
And yes, Robert Prosky of Hill Street Blue fame, JoBeth Williams of Poltergeist fame and Tucker Smallwood, from among other roles, God on the Sarah Silverman Show fame, all got their start at this locally produced Boston show. There was another cast member...Katrina I believe, but can't remember her last name.
Definitely outdated by today's standards, but had a certain charm nonetheless...