Recently upgraded work computer to Dell Latitude E5550 running Win 8.1. Standard work email environment is MS Outlook using outlook.office365.com IMAP server. However, I have always preferred to use Thunderbird for my day-to-day email needs, and with all previous work computers I have had no complaints with TB. My last work computer was a Latitude E6430 (dual-core, same as the E5550) running Win 7 Enterprise. With the E6430, TB worked perfectly, and I could manage emails on the server folders as well as a couple of GB of downloaded/saved emails in local folders with no performance problems at all. This has completely changed with the current E5550/Win8.1 setup. I have tried various fixes, but none have improved performance. I have even totally uninstalled the Symantec Endpoint Protection to see if ridding the system of that would help, but the TB performance remains abysmal. My current TB setup is as simple as possible -- only accessing the IMAP server folders. Here is a summary of how TB operates on this machine: 1) When I launch TB, it takes between 35 and 45 sec before a beginning TB window opens on the screen. During this time, TB is consuming 30-35% CPU (as indicated by Task Mgr). It takes another 10-20 sec before any TB GUI detail begins to appear in the TB window, and the "still loading" icon continues. By about 90 sec from initial launch, the "still loading" icon stops, and TB is displaying the server folders and it is possible to select a folder and scroll from email to email. However, there is about a 1-2 sec delay in scrolling between emails. This responsiveness is totally different from what I experienced with the previous computer. 2) The TB CPU remains at 30-40% up until about 90 sec from launch, then drops to about 1-10%, peaking up momentarily as browsing between folders and emails is done with the TB GUI. Browsing between emails is slow. 3) When I close TB, the TB GUI window closes immediately, but the CPU usage pops up to 30-35% and remains there for about 20 sec after shut-down. 4) The above summarizes my most recent setup -- accessing just the IMAP mail folders, where there currently is a relatively small amount of emails. I have tried other TB "profiles" as well, with one of then accessing just the downloaded/saved emails I have locally on folders organized by date. The TB performance in those setups is basically the same as described above. This is my first experience with Windows 8.1, and it may be that Win8.1 is doing something crazy with TB. All other applications on this computer seem to work with perfectly acceptable performance, however. Can anyone suggest what may be causing this weird TB behavior and performance?