Did you read the article? It seems like they had a plan to make him CEO, he got sick, they quickly found an interim CEO, and the moment he got back:

On the day Teixeira returned to his job, it's claimed, he was instructed to lead a company-wide layoff of 50 people, 40 of whom were in his MozProd organization.

Followed by:

"Mr Teixeira had ethical concerns regarding the layoffs because they were primarily motivated by a desire to increase profit margins at Mozilla, which was already operating at a profit," the complaint claims. "Mr Teixeira viewed this as antithetical to Mozilla’s values as espoused on their website: 'We're backed by a non-profit, which means we prioritize the interests of people first, not corporate profits.'"

They continue to retaliate against him by denying him bonus, and trying to maneuver him into a demotion. They even had the shitty audacity to say like "well this frees up time for your cancer treatments" which at that point he wasn’t getting anymore.

The complaint claims that Teixeira, appointed in August 2022, helped reverse the decade-long decline of Firefox, which generates about 90 percent of Mozilla's revenue and is the company's only profitable product. He's further credited with growing Mozilla's advertising business, and AI capabilities, and with reducing investment in the money-losing Pocket service.

Sounds to me like they’re just being really shitty to this guy who has done a lot for the company in general and was on his way to CEO before the poor behavior of these two (Chambers and Chehak).