Update2:
For giggles, tried to login from IE instead of Firefox and I'm having absolutely no problems. While logged in and successfully viewing wp-admin in IE, switched back to Firefox and still getting the 429 error.
Something's definitely wrong between Firefox/WordPress/X10Hosting.
Update:
From my Android phone, I was able to use the Forgot Password link on the wp-admin page. That sent an email to my registered email account with a reset password link in it.
From the link, I was able to reset my password to the exact same thing AND login to WordPress to Add a New Post.
I...
I'm having the same problem on my site too.
I've seen some responses about "As a result of an excessively large number of requests your website has received recently, we added a temporary rate limit to your website. We have now removed the rate limit, and you should no longer see the 429 error...