As you may have read already, Firefox 3.0.6 was released on Tuesday. Among the fixes introduced in this version is one for accessibility. It has to do with CaretMoved events and selection changes not being communicated to screen readers properly in the past.
One immediately visible change is that, when you open ChatZilla, the accessible IRC client extension. Previously, your screen reader would have told you erroneously that your focus was in the textbox where you type your message, when it was actually on the output HTML content.
Also when selecting text, you should now get more accurate results under many circumstances than before.
These changes should have a positive impact on all supporting assistive technology.