In December 2005, "Internet Week" ran an article in which many readers reported high memory usage in Firefox 1.5. Mozilla developers said that the higher memory use of Firefox 1.5 was at least partially due to the new fast backwards-and-forwards (FastBack) feature. Other known causes of memory problems were malfunctioning extensions such as Google Toolbar and some older versions of AdBlock, or plug-ins, such as older versions of Adobe Acrobat Reader. When "PC Magazine" in 2006 compared memory usage of Firefox 2, Opera 9, and Internet Explorer 7, they found that Firefox used approximately as much memory as each of the other two browsers.