Testing is only a priority on closed source apps - Windows and IE being no exception. The very fact that users have neither access to the source code nor the ability to build the application sources means that any testing must be done "in-house". This is going to slow down the release cycle by exactly the amount of time it would take to run all the regression tests.
With Open Source, a patch can be released right away and users can compile in the new sources themselves. Any issues can be immediately identified and reported back to the maintainers, often with both the offending source code and potential fixes to the patch. Without the lengthy QA cycle, Open Source patches are much more immediate than any Closed Source shop could ever hope to achieve.
What's that I hear? Why, I do believe it's the sound of people stampeding to replace their OSS products with MS stuff after hearing how OSS (or at least this guy) does business. You just became MS's MVP of the week, dude; congrats!