One thing that I am very happy about was a bug I noticed regarding the rendering of composed div elements that call dynamic code using AJAX. In my collaboration UI , a contacts list is displayed using composed AJAX calls, up until firefox 3, the display of the contents in this dynamically loaded section was irratic, depending on the amount of content loaded sometimes the outer call would resolve before the inner content had finished rendering. Thus, the outer call would render as if there was no code inside it, the pane displayed in a collapsed state instead of expanded. I could trick it by refreshing the page several times and get it to expand, but often the next manual page refresh would recollapse the pane. I tested the issue in IE , Opera and Safari (on an IPhone!) and none of them exhibited the pane collapse bug I encountered so I was sure it was a rendering issue that Firefox had. I am happy to report that the bug is now gone in Firefox 3, logging in to an account reveals the expanded display of the users contacts as it should, following links on the page refresh the page maintaining the expanded state of the pane exactly as it was supposed to. I am not sure what change was made by Firefox to fix this (probably some issue related to caching and rendering of dynamically populated div's) but it is indeed fixed!
Shot out to the firefox 3 team for relentlessly hunting down bugs and memory issues, I believe that well designed software should get tighter with each revision and the moz dev guys seem to follow that mantra as well. Kudos guys.