Opera, at least version 12, is sometimes detected as Firefox as Opera can claim to be Firefox. Here the user agent string:
Mozilla/5.0 (Windows NT 5.1; de; rv:2.0) Gecko/20100101 Firefox/4.0 Opera 12.01
Also I think the Opera Version are not correctly detected. Here another user agent string:
Opera/9.80 (Windows NT 5.1; U; de) Presto/2.10.289 Version/12.01
I think Opera version is detected as 9.80 instead of 12.01.