Have two profiles on Thunderbird, "Default" and "Other". Using Default, when you click on a link in an email, all works well--it opens a new Firefox instance, if not already running, or opens a new tab if there is a Firefox instance already running. Using the Other profile, when you click on a link in an email, it opens a new Firefox instance, if not already running, and gets an error if there is a Firefox instance already running (does not open a new tab): "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window." It seems to want to open new instance of Firefox instead of opening a new tab like the other profile does. This has been an ongoing problem for many months now and has propagated to a new computer with new OS and installs. Have looked a the many postings on this error and none seem to really fit this issue and none of the resolutions worked. Running Win10 and the latest versions of Firefox and Thunderbird. Firefox is the default browser and Thunderbird is the default mail app. Thanks.