I know of no way to do exactly as you wish, but the closest is maybe to add Reply, Forward, Delete and Junk buttons to the Mail Toolbar (right-click, Customize). These actions do not require the Message Pane to be open, and they can be applied to multiple messages. The same buttons can be added to the Header Pane at the top of the Message Pane for action on a single message.

I feared so as I read about your suggestion elsewhere already earlier. What I don't understand is why this hasn't been addressed long ago. Imagine if you had to handle just 50 mails per working day this would result in 250 per week, 1000+ per month and 12000+ per year. So if I don't handle the messages immediately while open in the reading pane or already selected (but let's say an hour or a day later) there's a potential of at least 12000 unnecessary clicks per year. Based on information found here (https://goo.gl/CVAY5H) it takes about 1.3 seconds to move, click and release (average) each time. That's a potential 15600 seconds or 4.3 hours wasted per year ;) Ok, worst case, but still annoying...

But hey, never mind and thanks lot for your quick reply!

You probably spend more time than that waiting for windows to start. Really time and motion studies have long proved that the greatest time waster in a days work is personal time taken by the employee. I really think saving 50 seconds a week to be a poor reason for change.

Yes, and waiting for Windows is also an annoyance. Ignorance, however, is a poor reason for no change. I just don't understand why UI developers can't follow the least possible click philosophy. It's a mail client for God's sake...

you could file an enhancement bug. See https://bugzilla.mozilla.org/