have you modified ccleaner as I discuss here http://thunderbirdtweaks.blogspot.com.au/2012/06/thunderbird-and-ccleaner-or-my-settings.html it is an oldie but a goodie. Perhaps you could tell me if the ccleaner folks have amended their ways over the ensuing years.

I am guessing the issue may be your network adapter. Did any browser other than edge work in Windows 10.

This is the article I have seen refered to for Windows 10. But the netsh command used is available in windows XP, So I assume it works in all versions of windows really, unless the parameters supported have changed..

Thanks Matt

Have modified CCleanner as you suggest but as I've used it daily for years it seems unlikely to be the problem.

Have got Thunderbird to work normally by booting Windows into Safe Mode with Networking. But I don't know how to drill down to find what is getting in the way with a normal boot. Any ideas would be appreciated.

Have looked at the article re netsh and shall go back to it if I have further problems with Win 10. Thanks again

If all works as intended in Windows safe mode it is almost always an anti virus issue. In this case probably a firewall component blocking access, or a mail checker that is basically broken.

So check the firewall and make sure Thunderbird get access to everything (be careful about version numbers. Nortons in particular will allow 30.1.0 but not 38.1.1 type variations) Make setting something other than Automatic.

Disable mail scanning. It carries very very little risk and see if that helps.