hcaptcha antispam challenge spinning the wheel indicating working, but developer console (F12 key, Console tab) displayed: "Failed to create WebGL context: WebGL is currently disabled." Opening new tab with address "about:config" and searching for webgl i have found "webgl.disabled" variable is set to true (non default status), so i revert it to false. Then reloading page and retry worked and error was gone. If this was done by some extension, i am wondering how to discover which one.