The User Agent starting with "Mozilla/5.0" does not mean that you are using Firefox. Almost all browsers spoof this part of the User Agent. Here is an excerpt from https://webaim.org/blog/user-agent-string-history/ (read this for more information):

"And thus Chrome used WebKit, and pretended to be Safari, and WebKit pretended to be KHTML, and KHTML pretended to be Gecko, and all browsers pretended to be Mozilla, and Chrome called itself Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.13 (KHTML, like Gecko) Chrome/0.2.149.27 Safari/525.13', and the user agent string was a complete mess, and near useless, and everyone pretended to be everyone else, and confusion abounded."

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