1) Edit-->Settings.
2) Search for Config.
3) About the "third or so search result down"...on the right side, click the button that says Config Editor.
4) In config Editorr, search for dir
5) Scroll down through results. There aren't too many. Find the ones you want to change and hit Edit. I suggest changing "mail.compose.attach.dir" to wherever your outgoing attachments will be and "messenger.save.dir" to where your saved/downloaded attachemnts will be.
6) Type in the long/linux directory name to the box and hit Enter to save it. Don't use the / character at the end of the directory name you enter. Once you hit Enter, the entry will look boldfaced now if it didn't already.
7) Exit Thunderbird completely and restart it.
Go to an email attachment. On the right side of the attachment name (at the bottom), click the Save pulldown. Be sure the directory is indeed coming up correctly. If not, go to the right directory and save it anyway. It will likely fix your problem (if you've done all above). Note that right clicking on name shows that Open and Save are different. This is changing it so that it Saves always. You may not want that. Up to you. See Step 11 either way.9) Assuming it is okay, click on *the name of the attachment* and Save a file once to be sure it saves in the right spot. If not, something is wrong above.
10) If it did save in the right spot, go in and click the name of the attachment again. This time Save with "Do this automatically..." clicked on so it always does it in the future.
11) You can play with what different types of files do by the following:
a) Edit-->Settings
b) Search for Content Type
c) Change "Save Files to" ...to your Download folder
d) In the little chart shown, you can also add certain file types to Open instead of auto-save. For example, always open PDF and Excel Worksheets.
It should now be working. Just click the name from now on.