“We are getting ready to release Firefox 1.0.6, Thunderbird 1.0.6, and Mozilla Suite 1.7.10 to address some API issues in the Firefox and Thunderbird 1.0.5 releases, and we want to gather feedback on extension compatibility. There is a very real chance that some of the general security improvements may impact a number of extensions that worked with 1.0.4 and earlier, and we want to identify and address as many of these as possible before we release 1.0.6.”
A new release so soon after the 1.0.5 releases of Firefox and Thunderbird is due to the problems they caused with extensions (ran into them myself, too).