Stylish 1.2.1 which will work with Thunderbird (and Firefox) 6.0 is currently stuck in the AMO approval queue, so users who are updating to Thunderbird 6 are finding that Stylish 1.2 is being disabled. User can install Stylish 1.2.1 via this link, just click the yellow Download Now’ button and click through the warning. Looks like they submitted the updated on August 15th, the day before Thunderbird 6 was released. Likely will be approved by the end of the week and those have add-on update checking enabled will see it as an update when they restart Thunderbird.