$ sudo strace -u ubuntu -f firefox

...

geteuid()                               = 999
openat(AT_FDCWD, "/", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY) = 4
mkdirat(4, "home", 0755)                = -1 EEXIST (File exists)
openat(4, "home", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY) = 5
close(4)                                = 0
mkdirat(5, "ubuntu", 0755)              = -1 EEXIST (File exists)
openat(5, "ubuntu", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY) = 4
close(5)                                = 0
mkdirat(4, "snap", 0755)                = -1 EEXIST (File exists)
openat(4, "snap", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_DIRECTORY) = -1 ENOTDIR (Not a directory)
close(4)                                = 0
write(2, "cannot create user data director"..., 65) = 65
write(2, ": Not a directory\n", 18: Not a directory
)     = 18