Change to appearance and folders gone missing

Over the last couple of days the general appearance / layout of Thunderbird has changed for me. I have attached a screenshot of how it appears now, previously under the 2 talktalk e-mail addresses I had a number of additional folders I also, under SPAM, had spam folders for each e-mail address, in fact I had 2 for each of the talktalk addresses. I first noticed the changes yesterday but after shutting Thunderbird down I got most of the old layout back, but today I see only what is in the image. Is this some sort of an update or has my installation been corrupted in some way.