excuse me. I was browsing in a website. and after that, there is a pop-up like it
[some text] <div id="extwaiimpotscp" style="display:none" v="{9225" f="ZXpreU1EUTNNamM4TFRBNU1UQXROW0ZpWWkxaVpZSTNMV0UzWmpKalpEWmpfakEwWW4wlQ==" q="b05a3bb7" c="57.45" i="60.47" u="10.30" s="19188318" w="false" m="BMe=" vn="yote2"></div>
is that XSS? and can I use it for steal cookies? and how to use? and what is the name of this code?
This is a firefox addons probably causing this html to be appended in your code. You can try disabling some addons. Though, it's clearly not harmful for sure.
https://quick-geek.github.io/answers/887067/index.html for reference.