[Solved]: How to prevent Thunderbird launching multiple Firefox windows

My default Web browser is Firefox and default mail user agent is Thunderbird.

Firefox has been set to:

“Open new windows in a new tab instead”

Despite this, every time I click on a link in an email, a new Firefox window is opened. I do not know if this is a separate instance of Firefox, but I suspect that it is not. This behaviour of Thunderbird is new to me and I think some setting might need to be changed.

When a new link is clicked in the browser, however, it is opened in a new tab as expected.

What should I do to make all new links—whether clicked from browser or mail user agent—open in a new tab within an already open Firefox window?


Following the advice at


I was able to get it to open in a tab of an already open window.

For some reason the field


[/b]in about:config with filter link.o

had been manually set to 2.

Resetting it to the default solved the problem.

For convenient reference, a screenshot is attached of the settings to prevent launching of multiple firefox windows.