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I'm using 48.0 and haven't seen that on a normal browser. However, the purple screen does show up if I open a private browser.
I also had a popup indicate that there was a critical Firefox update available last night. Something about it didn't feel right. It opened in a new tab and referenced a URL that I'd never heard of. WHOIS.COM hadn't heard of it either, which suggests that it was a cleverly disguised scam. Had I gone forward with what claimed to be a Firefox update could have been anything.
Are you sure that you haven't installed something malicious?