Cunningham and Costa, who worked at Amazon for 15 years, said they were fired via phone on Friday afternoon. The employees said the action was in retaliation for the continued criticism of Amazon and after sharing a petition from Amazon warehouse workers highlighting their concerns.

The spokesperson added in a statement that it supports workers' rights to protest and criticize their employer's working conditions, "but that does not come with blanket immunity against bad actions, particularly those that endanger the health, well-being or safety of their colleagues."

Amazon also confirmed Tuesday that it fired a warehouse worker in Minnesota who had spoken out about his facility's treatment of employees who continue to come to work amid the coronavirus outbreak. An Amazon spokesperson said the company fired the worker, Bashir Mohamed, as a result of "progressive disciplinary action for inappropriate language, behavior and violating social distancing guidelines." News of Mohamed's firing was first reported by BuzzFeed News .

The company confirmed to CNBC that it fired Emily Cunningham and Maren Costa, both of whom were user experience designers, for "repeatedly violating internal policies." Amazon's external communications policy prohibits employees from speaking about the company's business without approval from management. News of Costa and Cunningham's firing was first reported by The Washington Post .

Amazon fired two employees late last week who were outspoken critics of the company's labor practices, including, most recently, its treatment of warehouse workers during the coronavirus outbreak.

Costa has publicly shown support for warehouse employee advocacy groups, recently tweeting that she would donate $500 to support fulfillment center workers on the front lines. In a statement, Costa said Amazon attempted to intervene in her attempts to set up meetings between tech workers and warehouse employees to "hear what it is like working in warehouses during the coronavirus."

"Why is Amazon so scared of workers talking with each other?," Costa said. "No company should punish their employees for showing concern for one another, especially during a pandemic!"

Cunningham said in a statement: "When warehouse worker colleagues asked us for support to get better coronavirus protections, we knew we had to do something. Warehouse workers are putting their lives on the line and are under real threat right now. We have to do all that we can to support workers on the frontlines, now more than ever."

Cunningham tweet

Costa tweet

Both of the employees are members of an employee advocacy group, Amazon Employees for Climate Justice, which has called for the company to develop a comprehensive climate change plan, among other environmental initiatives. At Amazon's shareholder meeting last year, Cunningham called for Amazon to reduce its use of fossil fuels. Costa and Cunningham also appeared in a video with Sen. Bernie Sanders, a frequent Amazon critic, to criticize Amazon's climate stance.

Amazon previously threatened to fire Costa and Cunningham if they continued to violate the company's external communications policy. The policy has been a subject of criticism from employees. In January, more than 340 Amazon employees signed onto a letter in protest of the policy. In response, Amazon said it would continue to enforce the policy and would "not allow employees to publicly disparage or misrepresent the company."

The company's labor practices have attracted attention in recent weeks as warehouse workers have staged protests at Amazon facilities in Staten Island, New York; Detroit and Illinois. Amazon drew further criticism after it fired a Staten Island warehouse worker who organized a strike to demand greater protections for employees amid the coronavirus outbreak. Chris Smalls, a management assistant at the facility, said he was fired for organizing the strike, but Amazon said it fired Smalls because he violated social distancing rules while he was supposed to be under quarantine after being exposed to a coworker who tested positive for the coronavirus.

In addition to growing concerns from warehouse workers, Amazon has also faced criticism from union officials and legislators about a lack of protective measures for employees who continue to work during the coronavirus outbreak.