Comment 6 for bug 1774121

This is a problem on Linux too.
It seems the FilePicker in this case is called with Mode="select_folder" and then the native toolkit (GTK, QT, Win32,...) will pick the text on the button (Open). This mode is probably needed to allow choosing a folder only.

When saving one attachment, Mode="save" picks the correct text (Save). But it's aim is to choose a folder but also get a filename from the user (usually prefilled).

So it is probably necessary to keep both modes separate. I don't know if there are any other modes available from the toolkit which would help here.