There are native versions of Firefox and Thunderbird for Linux. Many distros ship with those already installed.
The “less popular” part is only sort of true - consider the wide array of distributions out there - its hard for hackers to count on anything being a certain way in Linux. But it is true - hackers take advantage of more ubiquitous OSes.
Also - its not about “coding errors” - Linux strength/security comes from the overall approach to OS architecture - particularly when it comes to userspace - (although I think newer Windows has improved here somewhat)
Thanks.