I initially upgraded a Sony VAIO VGN-NR498E from Xubuntu 14.04 64-bit to Xubuntu, which it had been using for the past two years, to Xubuntu 16.04 64-bit. Prior to upgrading the operating system, Thunderbird was fully functional on this laptop. After the upgrade, Thunderbird would launch, however it would not completely launch; it would not show the current email from the email data file, nor retrieve new email. Thunderbird stops at the point shown in the attached screenshot, and the "loading icon" spins continually. The same problem occurs on other computers in my home, all 32-bit systems, when Xubuntu 16.04 32-bit version was installed on them. This problem occurs regardless if it is 32-bit or 64-bit operating system. On my Windows 7 computers, where Thunderbird still works, a message regarding the program "building summary file for INBOX" appears. I do not get this message on the Xubuntu devices. (NOTE: I share a single email data file with all my home computers in my home network, providing access to the same email via a networked shared drive, regardless which computer I'm on. This has worked flawlessly for a couple of years using previous versions of Ubuntu, Xubuntu, and Windows 7. All computers in my home network, regardless which operating system is installed, are able to access all shared network drives in my home network and perform all functions other than Thunderbird normally.) I found if I had Thunderbird already open to the email account on either Windows 7 device, then it would launch in Xubuntu 16.04 32- or 64-bit to the point where I could read the email. I could also send email from the Xubuntu 16.04 computer once Thunderbird completely launched on it, however the sent email was not being stored in the SENT folder. In addition to this, I was not able to delete any email on either Xubuntu 16.04 device, regardless if Thunderbird was closed on the Windows 7 computer, so file sharing/ownership does not appear to be the issue. I then completely re-loaded Xubuntu 16.04 64-bit on the Sony VAIO, only this time I had it completely delete all previous files on the hard drive; what I refer to as a "clean load." The problem was not resolved, and remained identical to the above description. The email account being used for this uses a POP3 server, rather than IMAP. When Thunderbird is launched after it is already running on another device, and it retrieves the stored data files, it shows the message "building summary file for INBOX" in the status bar during the retrieving process. I can then read all the email once this finishes, however all the same problems as indicated above remain.