To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thanks Matt. In fact I found that a simple system reboot cured the problem.

Matt, any idea why a reboot would have helped?

Wayne Mery said

Matt, any idea why a reboot would have helped?

It is possible that the TCP/IP stack on the machine was corrupted and a reboot forced a refresh as the in memory stack was rebuilt. While Microsoft does rigorous testing on the robustness of their TCP/IP components. The same can not always be said of third party components installed by Anti virus program and other security and Intellectual property protection software.

All that is required for a TCP.IP stack to start coming apart is say a firewall that does deep packet inspection not keeping up with the workload . It can say manage 1mbs but the connections is 10mb and the connection is busy as the household streams video to the lounge, audio to the kitchen and another lot of media to a bedroom or two.

The trend to using interpreted languages (Usually very slow compared to Assembler) on low level components like firewalls and routers in an environment of ever increasing network bandwidth will probably see these network components get a lot of attention over the next couple of years. Just as replacing mechanical hard drives became a hot topic for upgrading to make a computer run exponentially faster in may cases.

On the flip side, it has been standard diagnostics/ repair for as long as I have used computers (going back to DEC minis and Wang minis running MVS to reboot before you do much else as memory corruption including on die memory, is not flushed without a reboot. While computers have become increasingly reliable, the size of the die has shrunk to the point where some of the electronic functions are almost quantum states. That is, gates can be on, off or both due to electron bleed.