Problem: newly added account disappears when TB is closed. Thunderbird on this PC (Dell Latitude 6540) has held a Verizon account for years. The "new" account is a different, equally old Verizon account kept on TB in an identical PC I no longer use. When added to TB on the present PC it tests and works fine until TB is closed. Repeated numerous times. Having read thread # 1286437 I did what follows , to no avail: 1. tried TB in troubleshoot mode; 2. tried in Windows safe mode; 3. cleared startup cache; 4. drastically pruned old emails; 5. looked up profile folder under troubleshooting info. Folder qxxxxxxd ( size 1.18 GB before adding new account) holds one folder tree, one directory tree, but NO prefs.js file. Instead, twenty-two prefs-xx.js files from 1 to 22; removed all to a thumb drive except prefs-1.js which I dared not rename as prefs.js; 6. found two profile folders, both labeled default. One is the above holding 1.18 GB, the other holds 685 MB (probably harmless and related to an old university account no longer extant). I copied both profile folders to a thumb drive. Note: profile folder qxxxxxxd is clearly engaged in what I am attempting. When TB is open with the added new account working -- which is the case as I am writing --, profile folder size increases from 1.18 to 1.20 GB and the number of folders increases by one (under "Mail" the list of pop.verizon-xx.net sub-folders increases from 17 to 18). Please advise on what to do next; I am 86 and no software geek, but can follow instructions... Thanks --Paul