User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.6) Gecko/20100625 Firefox/3.6.6 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.6) Gecko/20100625 Firefox/3.6.6 Please bear with me for an explanation, summary line is too short for that purpose. I am giving steps to reproduce for Firefox, but it is the same with Thunderbird (just set inbox folder to specific drive instead of profile). Reproducible: Always Steps to Reproduce: 1. Set Firefox profile folder to a specific drive (say E:) 2. Make sure Firefox is not running 3. Open command prompt window 4. Type: chkdsk /X E:<Enter> (this will force volume E: to be dismounted!) 5. Try to start Firefox Actual Results: Firefox cannot start, it shows the message that it is already running when it is not -- it is just unable to access the profile. Expected Results: Firefox should start and let the user know that the profile is (temporary) inaccessible, and perhaps offer an option to try again later or create a new profile elsewhere. On Windows platform, the usual way to detect another instance of a running application is to CreateMutex() (global, named) upon startup. If that fails with GetLastError() == ERROR_ALREADY_EXISTS, then the application is already running.

I'm certain there's already a bug on this - either in toolkit or core

This isn't just a simple bug, it is an improper way to detect running instances of an application for the Windows platform -- code for detection should be made platform specific and proper code should be added. I provided info above on how to do it correctly.

you are correct. there is a code problem. this is bug 278860