Mozilla programmers are gradually reducing the number of Firefox 4 bugs.

With plenty of competitors breathing down its neck--Microsoft's IE9 in particular--Mozilla is fixing the final bugs that lie in the way of a Firefox 4 release.

Last night, Mozilla released Firefox 4 beta 10 (click to download for Windows, Mac OS X, and Linux). This version focuses mostly on stability and performance rather than on new features--although one new option is the ability to start Firefox in safe mode by holding down shift when launching on Windows.

Flash should work better on Mac OS X with the new beta, and memory usage should be better, Mozilla said in a blog post yesterday.

Next up are one or two more beta versions, then the release candidates, and if all goes well, a final Firefox 4 release in February. In a planning message last night, release manager Christian Legnitto said Mozilla will build beta 11 and, if too many bugs remain unfixed, beta 12.

Mozilla recently divided its blocking bugs into two categories, soft and hard, with only the latter able to hold up release of the new software. As of Wednesday morning, excluding security bugs, there are 194 soft blockers and 76 hard blockers.

In practice, it won't make much difference whether Firefox 4 is released before Internet Explorer 9: most people don't change browsers that frequently. But competition is fierce right now, especially with Google's Chrome edging in on the early-adopter stronghold Firefox once had more to itself.

IE9, currently in beta testing, changes the competitive dynamic in the browser marketplace. Microsoft for years lagged rivals for support of new Web technologies. IE9 embraces many of them, though, and adds significantly improved performance at the same time. It's not clear when exactly IE9 will be released, but Microsoft's developer-oriented Mix conference in April seems a fitting venue.

One of the headline features in Firefox 4 is hardware acceleration using computers' graphics chips. However, Mozilla has decided to implement the feature cautiously to begin with to avoid crashes or other problems among its 400 million users. Specifically, Firefox 4 will disable hardware acceleration except with new graphics drivers and with a whitelist of video hardware makers: Intel, AMD, and Nvidia.

"We just want to minimize our initial risk here," graphics team member Joe Drew said in a mailing list discussion. Mozilla plans to remove the whitelist later, he said.