Firefox is my browser, but I also have Windows Office 365. Every time I open my my gmail account or anything else I have to reset Firefox as my browser. How can I stop that?

Firefox is my browser, but I also have Windows Office 365. Every time I open my my gmail account or anything else I have to reset Firefox as my browser. Windows keeps trying to change my browser. I use and have many documents on Office 365 so I don't want to delete it. How can I stop that from happening. I can't disable the Microsoft browser.