Every time when you solve CAPTCHA, the CAPTCHA answer can be used as a common secret for a short time. Mainly, It can be used to prevent TLS certificate spoofing.
Since ordinary web pages (and their JavaScript) doesn't have access to TLS certificate data, I was have to make this extension. Actually it does the following:
let clientsideDigest = <Digest of TLS certificate that you get from website you're visiting>;
let yourAnswer = <Your CAPTCHA answer>;
let resultDigest = PBKDF2(clientsideDigest, "SHA-512", yourAnswer);
//Where yourAnswer is PKBDF2 salt.
cookies["TLSCaptcha"] = representAsHexString(resultDigest);
To see it in action, you need to visit a website that have server-side implementation of this scheme.
At the moment of publishing this extension, there is no server-side implementations. If you want to make your own, please look into source code for details.
New additional featue: you can use this addon to establish additional encryption. See source code for details.
WARNING: this extension cannot be ported to Chromium-based browsers due to API limitations.