I have a client who recently updated to the 45.1 release and is now having trouble sending emails. After some remote troubleshooting, I found that when first opening Thunderbird and then clicking 'Write', the first instance seems to hang when clicking 'Send'. Close the first instance, then subsequent emails get sent as normal. I hope I've explained this clearly; I found a similar problem mentioned in the support forum but that one implied no emails were being sent at all.