Every file I open from Thunderbird is opened in read-only mode.
I suggest an option similiar to Excel behaviour, to DISABLE read-only mode and open read-only files directly in EDIT MODE (as file copy).
Of course, when you try to save them it will open "Save As" dialog.
We have too many options, but I'll set to NEW anyways.
There are lots of bug reports providing cases where it would be very bad to open a read-only file as "untitled", so the suggestion here is not a universal fix. This particular case (file saved in temporary folder as read-only file most likely) currently will prompt for a new filename/folder already as soon as you press save, which is always a good idea to do first if you want to edit a document that is sitting in a temporary folder.
The workflow for this particular situation of email attachments/temp folder documents has changed in recent years, so this could be marked as either WorksForMe, or WontFix.