I see what you mean. Selecting across the whole subject with the mouse also doesn't stick.

But I found a workaround: if you click to the right of the x which is at the far end of the line and drag to the left toward the x, the subject gets selected and doesn't clear.

It's not perfect: when you copy, you get the tooltip for the x button, but it's better than nothing!

Thanks for the reply, I found I can also double click and hold it down and sometimes it works but unfortunately I only need to grab 1 or 2 words from the subject to copy and it is tough to keep doing that. It worked perfectly up until last week, It works on chrome and IE just not on Firefox. Thanks again and hopefully somebody can fix that.

If you find it frustrating enough, you might post it on Microsoft's forums. They might not always be testing their product in Firefox. ;-)