Mozilla's Firefox fell short of breaking the 20 per cent market share bar in July, a Web metrics firm said Tuesday, as the open-source browser's growth slowed dramatically.

Rival Internet Explorer (IE), however, held its share for just the third time in the past year, while Apple's Safari posted its largest market share dropoff since June 2007.

According to Net Applications, Firefox accounted for 19.2 per cent of the browsers used to access the 40,000 sites the company monitors for its clients, an increase of just 0.2 percentage points over June. The gain was substantially lower than the 0.7- and 0.6-point increases that Firefox posted in May and June, respectively; it was also less than half the average increase during the last 12 months.

"I thought Firefox would hit 20 per cent by July," said Vince Vizzaccaro, Net Applications' executive vice president of marketing, referring to a prediction he made two months ago when Firefox's trend line was steeper. "Now it looks like that could be still be another couple of months off."

Use of Firefox 3.0, the major upgrade that debuted in mid-June, however, continued to grow; the new version's share climbed from 2.3 per cent at the end of June to 5.7 per cent at the end of July. Most of July's gains, however, appeared to come at the expense of its predecessor, Firefox 2.0, which lost 3.1 percentage points last month.

The move from Firefox 2.0 to version 3.0 will likely accelerate when Mozilla decides to automatically offer the latter as an upgrade to Firefox 2.0 users. Although the company has discussed the move, it hasn't yet set a date when it will trigger the update offer.

Microsoft's browser, meanwhile, held steady in July at 73 per cent market share, the same as in June. IE has lost share in nine of the last 12 months, and on average slipped by 0.6 per cent during those months.

Safari also ran counter to normal during July. Apple's browser usually posts small gains each month -- primarily on the back of increases in Mac OS X market share -- but the application was off 0.2 percentage points in July, the biggest drop since the 0.3-point decrease in June 2007.

In other data, Net Application said that Mac OS X's share of the operating system market was also down 0.2 points from June.

"That could just be a little blip," Vizzaccaro said. "They normally don't have a setback, but this may mean its share might grow more slowly."

Net Applications' browser share and trend data is available online.