Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

I also have this problem, and have just gone into safe mode to see what happens.

And within a minute of my answer above, I opened a few more tabs, closed one without incident, and then closed another, after which the window shrank horizontally from full screen to half screen. Problem not solved.

This has been happening for several months, through several versions. Currently running 4.0.2 in Windows 10.

pedagogue said

And within a minute of my answer above, I opened a few more tabs, closed one without incident, and then closed another, after which the window shrank horizontally from full screen to half screen. Problem not solved.

Is it exactly half the screen, like what happens if a window is maximized and you hold down the Windows key and press the left arrow key, or "restored" to the default resizable window size? Either would be weird...

jscher2000 said

pedagogue said

And within a minute of my answer above, I opened a few more tabs, closed one without incident, and then closed another, after which the window shrank horizontally from full screen to half screen. Problem not solved.

Is it exactly half the screen, like what happens if a window is maximized and you hold down the Windows key and press the left arrow key, or "restored" to the default resizable window size? Either would be weird...

For this question, it does look like exactly half the screen. I have a separate thread with the question "Why does the entire firefox window frequently shrink when I close a tab." Your are making suggestions on that thread, jscher2000, so I will stick to that and drop out of this one