Question: I do not use the default Mail.app from Apple. I prefer Mozilla Thunderbird. Can I uninstall it to save space and reduce clutter on my Mac?
Answer: I personally agree with the principle. Mail.app is a decent default option, but there are other email solutions out there and I believe that many are superior. That doesn’t mean you should remove it from your Mac, however.
It may be a bit of a paradox but the very reason why you want Mail.app to stay around is to preserve your ability to control easily which application is your default for email.
As you can see above, preferences in Mail.app help you to choose the “default email reader”. There are other ways, such as modifying a preference file, but doing it from Mail.app is the easier, graphical, safe choice.