User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2) Gecko/20070222 SeaMonkey/1.1.1 Mnenhy/0.7.5.0 Build Identifier: Thunderbird version 1.5.0.10 (20070221) When updating Thunderbird from one version to the next via [Help > Check for Updates], the progress popup should contain a Cancel button that is enabled until downloading is completed. Once actual updating begins from the downloaded MAR file, then the Cancel button should be disabled. Reproducible: Always Steps to Reproduce: 1. With the immediately prior version of TBird installed, request [Help > Check for Updates] on the menu bar. 2. Proceed to initiate the update to the latest version. 3. Selecting the X to close the windows (upper-right corner), terminate the progress popup and TBird. 4. Restart TBird. Actual Results: The download and update process resumes from where it was interrupted. There seems to be no way to really stop the update process without (in Windows) resorting to Ctrl-Alt-Del, locating the process, and killing it. Expected Results: There should be a Cancel button on the progress popup. Closing both the progress popup and TBird itself should also terminate the update process. In all cases -- for any application -- the user should be able to cancel a download and subsequent update, at least before it has progressed to the point where a corrupted application would result from cancelling.

Can anyone explain how a Thunderbird bug report can be resolved as a duplicate of a Firefox bug?

Because the software update is shared code.