Lack of support for wildcard certificates is still an issue for Let's Encrypt. Rate limiting and SNI are two issues that means a wildcard certificate is still highly desirable.
Let's Encrypt actually has compatibility issues with Windows XP, so wildcard certificates wouldn't help.
https://community.letsencrypt.org/t/which-browsers-and-opera...
But the biggest problem here is wildcarded subdomains, since LE doesn't let you get more than 5 certificates per domain.
Also I think with Firefox on Windows XP it should work.