If you don't have the have the above preference set, check your list of add-ons for an extension that modifies the user agent string. If you've double-checked neither is the case, feel free to reopen.
Bug 1613290 Comment 2 Edit History
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If you don't have the have the above preference set, also check for *general.useragent.override* and look through your list of add-ons for an extension that modifies the user agent string. If you've double-checked neither is the case, feel free to reopen.