Does the same thing happen in safe/troubleshoot mode (hold Shift when launching TB)? On my setup, the filter window is normal-size, but I notice it can't be resized by click and drag at the corners.

OK, I started in troubleshoot mode and I still have the issue.

If this happened to me, without any obvious fix, I would create a new profile, add an account, and test the filter function. If it works properly, data can be transferred from the old profile to the new. Help/Troubleshooting, about:profiles, to create and manage profiles. This advice is for users who are comfortable managing multiple profiles.

I'm an old guy and I'm not comfortable doing that. Do you have anything else? What's the best way to see if anyone else has this issue? Kinda ticks me off that the update caused this!

I can't find the option Create Filter From Message at all! Used to be there before updating to Supernova, now it's lost. Has it been moved?

I'm seeing it. With an email open, I clicked "Message" near the top of the window. When the list pops up, keep scrolling down to Create Filter near the end of the list. A Create Filter window opens, but it's totally useless as the window is too large and the button to click to create filter is out of view at the bottom of the window. The window is not able to be resized. Thanks Thunderbird for making this feature unusable!

Thank you. I was looking for it in the menu (Tools seemed a sensible place to look into). However, in my case the window works fine and I can see the buttons correctly.