Can't move the separator between the message list pane and message pane.
I have two TB instances; one for personal mail and one for my work. A few weeks ago I upgraded TB to 60.3.0. Since then, I was unable to move the separator bar between the message list pane and the message pane in the personal instance (both instances use the "classic" layout, with a folder list on the left and the message list and message on the right, with the message pane below.) The two panes were "stuck" in a position showing only three messages in the list. I've lived with that, but it's been a bit of a pain. (Note: I don't really know whether the upgrade caused the problem.)
Today, I upgraded to 60.4.0, hoping it might solve the problem. It didn't, but I got a "good enough" solution by changing to the vertical view, and then immediately back to classic. Now, when the mouse hovers over the separator bar, it changes to a double ended horizontal arrow. Ignoring the orientation of the arrow, I can move the bar up and down, which is what I wanted. (I say "good enough"; "perfect" would be that the arrow would be vertical.)
Wšykne wótegrona (2)
That sounds a lot like a problem with an add-on, or a theme.
So try restarting with add-ons disabled from the help menu and see if things improve. If they do post your add-ons and we will see if there is something about them that anyone knows about
When you start in safe mode, check 'Reset toolbars and controls' and click 'Make changes and restart'.