There are a lot of bugs in Firefox 4. Mozilla this week updated the next gen Firefox browser to Beta 10, quashing no less than 506 bugs. Taking a deeper look at the 506 bugs fixed in Beta 10 reveals a tonne of stability fixes. There also appears to be a number of memory fixes, which is also a good thing.
There is actually at least one new feature/usability improvement in Beta 10 as well. With Firefox 4 Beta 10 users can now double click anywhere to create a new tab. This is a feature that was proposed for Beta 8.
There are still a few more milestones officially left for Firefox 4 including at least one more beta and a pair of Release Candidates.