In a response to a support question, the Mountain View, Calif., company said AdBlock, which is used to block advertising on Web pages, "often interferes with Gmail's chat features, causing Firefox to crash."
Google said its engineers are working on the problem, but recommended, in the meantime, to disable AdBlock, clear the Mozilla Corp. browser's cache and then log back into Gmail.
"If disabling AdBlock resolves the problem, but you'd still like to use it in conjunction with Gmail, you may want to contact AdBlock's support team for help," Google said.
Google launched Gmail Chat on Monday. The IM software is embedded into Google's existing email service and is being rolled out gradually among Gmail users.