Comment 45 for bug 78538

You second point is valid. But let me explain something. I don't see any conceptual difference between having multiple workspaces and multiple X screens. So if a user has FF in one workspace and Thunderbird in a second, clicks a link in Thunderbird and FF (for some reason, see later) doesn't steal focus, the user will be equally confused. So the whole user experience goes and falls with FF's ability to steal focus and make itself visible if the user opens a link.
If FF doesn't steal focus, and the user isn't aware that he has multiple workspaces, he will never find the FF window. Equally, if the user isn't aware that he has multiple screens, and FF won't steal focus, he won't find the FF window either. And I think you'll agree with me that it's more likely that a user (who, for example, comes from the MS world to Linux) isn't aware of workspaces than a user not being aware that he has multiple screens (imagine that!).
I don't know whether the focus stealing is configurable or not, because it has happened to me more than once that I clicked a link and FF _didn't_ steal focus, and I then was confused because FF was already running in a different workspace! Maybe it was a bug or some other reason, but it definitely wasn't nice. Someone familiar with the 'focus stealing' code could shed some light onto how FF behaves in that regard.

To your first point, it's hard to now decide what the proper fix is. Some users should come up and comment on how they want FF to behave. I thought about openURL(new-tab|new-window) and adding new flags like same-display, default-display or display=:0.1 etc to allow flexibility. I definitely would miss the feature that FF uses the existing window even if it's on a different screen!