Her family shared the sad news over the weekend, saying she'd passed Friday following a surgery to remove a cancerous mass from her liver ... which her nephew, Brad Zitzner, says resulted in her catching an infection shortly thereafter.

By all accounts, she fought hard ... but lost her battle to cancer. Right before going in for the procedure -- Thea's brother, John, posted a video of her asking for well-wishes from fans (young and old) which she said would help her in her recovery. She seemed to be talking in the Muriel voice, too, from the famed Cartoon Network show that she's most famous from.

John posted yet another video on Facebook, which seemed to indicate this is something Thea has been dealing with for a while -- apparently, she'd gone in for an initial surgery just a week ago ... and he has a video up of her talking following that procedure.

Her nephew writes, "Thanks Feefer for your leadership of your family. The role you have taken with my kids and all your family sadly can't be replaced. I can see hundreds of fan tributes already online so your love was shared far and wide."

Brad adds, "You loved life and even know I know the final outcome I love your decision to go for it and to try to win."

Thea hadn't gotten into acting until later in life -- it seems her first major role was on 'CTCD,' on which she starred for four years. She's also lent her talents for shows like "Pecola" and 'Scooby Doo' ... but it looks like she's been faithful to portraying Muriel in a variety of forms.

She was 81.