As you can see in the below screenshot, the sidebar version of Facebook Messenger is no longer usable due to several issues. Photos, images, profile pics, videos, etc. take an extremely long time to load - and many times won't load at all. But the biggest issue is the "user contact info/options" section on the right taking up the majority of the space, and reducing the actual messaging/chat-box section in the middle to a tiny, unusable size. The contacts list section on the left seems to be reduced in size as well, although that wouldn't be much of an issue if the profile pictures would ever load properly.
For now I just went ahead and disabled the Facebook Messenger sidebar extension since it's unusable anyway. It's a shame that it doesn't work properly, because having FB messenger in the sidebar is actually one of the better ideas the Opera Devs came up with. But I mean if it's not usable, then there's no point in keeping it. I hope the Devs get this issue fixed. It doesn't seem like it would be all that difficult to do.