Despite having been launched over a week later than IE9, Firefox 4 surpassed its rival's level of usage. While IE9 was used for 1.04 per cent of the world's browsing in March, statistics from NetApplications showed Firefox 4 accounted for 1.7 per cent.
Analysts suggested the primary cause for the relative decline of IE9 in comparison with Firefox 4 seemed to be IE9's incompatibility with Windows XP - still the operating system for most business users.
Another growing trend is the growth of the third and fourth browsers on the market respectively Google's Chrome and Apple's Safari. Their share of the total browsing was still relatively far from the two giants, but continued to grow exponentially, contributing to a much more fragmented market.
But Microsoft is unlikely to be quaking in its boots yet. When all its active browsers were added together, IE market share hit almost 56 per cent, whilst Firefox lagged behind at just over 20 per cent.
Read our reviews of Mozilla Firefox 4 and Microsoft's IE9 to see what you think.
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