Microsoft has since yanked the blog post by Penn that outlined the bump in Bing usage.
Because Bing is tightly tied to Edge -- and baked into several other parts of Windows 10 -- changing the browser during a Windows 10 upgrade also favors Bing, to the detriment of rivals, primarily Google.
Along with the public displeasure at Windows 10's behavior, Mozilla also posted step-by-step instructions for users who wanted to restore Firefox as the default browser. Those same tutorials could be used to make Chrome or Opera Software's Opera the default.
"We strongly urge you to reconsider your business tactic here," Beard told Nadella. "We are ... again insist[ing] that Windows 10 make it easy, obvious and intuitive for people to maintain the choices they have already made -- and make it easier for people to assert new choices and preferences."
Microsoft did not immediately reply to a request for comment on Mozilla's complaint.
Sign up for CIO Asia eNewsletters.