The communications company - the largest ISP of its type in the US - has reportedly revamped the software that users need to install to use its services.
According to security researcher Brian Krebs, the limitation affected one of his friends who signed up for the Comcast Xfinity high-speed internet service, who discovered what he initially thought was the effects of malware on his Apple Mac.
"Something had hijacked his internet settings. The technician who arrived to turn on the service said that a software package from Comcast was necessary to complete the installation", said the security researcher's friend.
Krebs says that his friend later discovered that his home page had been changed to comcast.net, and that the Comcast software had modified his Firefox profile so that there was no way to change the home page setting.
"I contacted Comcast; they initially blamed the problem on a bug in Firefox. Mozilla denies this, and says it's Comcast's doing", says the security researcher in his latest posting.
"This is NOT a Firefox bug or issue", a Mozilla spokesperson wrote in an email. "It is a Comcast method that applies preference changes to Firefox."
Comcast has now acknowledged the issue, says Krebs, adding that the problem is limited to Mac users, and that permanency of the change was unintentional.
A spokesperson for the communications company told the researcher that the company is in the process of correcting the installation software.
"Customers absolutely should be able to change their preferred homepage anytime", a spokesperson told Krebs. "We're obviously apologising for any inconvenience we've caused Mac users."
Fortunately for Comcast users, there is a DIY fix for the issue, and communications blogger Ryan Parman has published step-by-step instructions and screenshots showing how to remove the home page hijack.