I was trying to download burp's CA certificate for Firefox on Kali from http://burp as specified in its documentation but the site is not available. It redirects me to http://www.inert.com/. So is there any alternative to download the certificate?
You need to make sure you are using BURP as your proxy in your browser first. It sounds like you aren't doing that.
From their docs:
If you have not already done so, configure your browser to use Burp as its proxy, and configure Burp's Proxy listener to generate CA-signed per-host certificates (this is the default setting).