Try ctrl+left click.

Many Thanks for the suggestion. It does work as a work-around but doesn't really solve the problem.

I was looking for something that would happen automatically, rather than for the user to have to remember to ctrl+left click. I would have thought that setting the target to "_blank" would have been a realistic option as that is how all other tab / windows would work so was wondering if there is an equivalent of "_blank" for Firefox to open a webmail in another tab (something like "_mail" for instance)....

I have the same problem. Writing an email is a completely different thing from browsing the web and often you will need information from the website that includes the "mailto:"-link to write the mail. Thus opening the mail client in the same window really is not an example of ergonomic GUI design.

Manual workarounds will not work as nowadays often mailto-links are scrambled in a javascript to prevent spam. The new tab/window will then show the javascript command in the address bar, which will, of course, fail to function.

Opening Gmail in a new window or new tab by standard when using it as the default mailto-client in Firefox would be most welcome.

BTW this is how it works Chrome by standard, too. And I actually do not want to switch back just because of the handling of mailto-links. But it is too annoying on the long run.