In order to open any file, it has to be saved to your hard drive, even for viewing only. In Windows that is placed in the temp folder. Your default is to open emails and save to desktop. However, you can choose where to save them. Thunderbird > Preferences > Attachments > Incoming tab select : 'Save files to' click on 'Browse' and choose where you prefer them to be saved. You could choose any folder but selecting a temporary folder may be preferable. Maybe this info will help: To save files to your temp folder: Press the Command-Shift and G keys all at the same time (the “Command” key is also known as the Apple or Windows key depending on the keyboard you’re using). Type “/tmp” (no quotes) into the box that comes up. Press “Go.” Select “Open.” Located some info from another question which may help on a mac: https://support.mozilla.org/en-US/questions/1169946 ''1) In the Finder, open the Go pull down menu.'' ''2) Select Go To Folder at the bottom.'' ''3) A window will open up. Enter /private and hit the Go button.'' ''4) Find the folder called tmp, and make an alias of it.'' ''5) Move the alias to your desktop.'' ''6) Go to any location you like (I went to Library/Applications) and create a folder. I called my folder Thunderbird Temp Space.'' ''7) Place the tmp alias in that folder.'' ''8) Open Thunderbird's Preferences and select Attachments.'' ''9) At the bottom of the attachments window, select the Save Files To button, and then hit the Choose button to tell it where to save the files.'' ''10) Find and select the tmp alias you created. You will now notice that the window next to Save Files To says /private/tmp.'''' ''Now when you open attachments, they will open in the tmp folder. The contents of this tmp folder is cleared on each reboot.''

thanks i will try that