See http://kb.mozillazine.org/Recovering_deleted_mail_accounts

in the steps skip straight to the import export tools, you don't want to recreate your account, just get the mail back into a local folder. (note you have to create and select the folder of the menu items for the add-on are not available.

I'm having trouble following your instructions. I downloaded and installed the import/export tools add on to Thunderbird but when I try to import the old mail folder all its contents are grayed out and no matter which import approach I try I get nowhere. It looks like the files are there -- though greyed out they are listed -- so I'm stumped.

Create a new empty folder - I suggest under Local Folders. Select it. Does ImportEXportTools allow you to select the item you want to import, now that you've told it where to import to?

Creating the new folder and selecting it did get Import/Export Tools to import what looked like everything I'd restored from the library of a date before I lost those emails -- but when I examined the contents all the titles were there but all greyed out and when examined were empty. When I go to time machine, select all the things I want from the library and restore them, it takes a while to do it and looks to be importing everything-- but if so, where is it? When I examine the mail folder in my library after the restore I see what looks like everything in textedit format and looking at an individual file I see the desired content along with a lot of other stuff. Is that format wrong for Import/Export Tools????