Do you have a small resolution screen? See https://support.mozilla.org/en-US/kb/small-resolution-screens for suggestions.

Thanks for getting back to me.

No, the screen resolution should be fine: 1366x768 on the laptop, 1600x1200 on my external display. The window can't fit on either.

I've figured out the quickest work-around:

Alt-Space, x

But this is still a work-around that really shouldn't be necessary!

I'm reluctant to mess with the screen fonts that the link suggests. I don't want my screen fonts to get any smaller!

I might try disconnecting the external monitor and re-booting. That shouldn't make a difference, but who knows...

Thanks, Mark.

What OS are you using? Some handle different resolutions better than others. I am using similar to yours on Ubuntu with no problems.

added.... I just saw above that you have Win 8.1 Does it work OK with just the native screen?

Thanks again for getting back to me, and so quickly!

I have now tried it with just the native screen, and, no joy, the problem persists.

If you click the Maximize button, does it fill the screen properly?

If you 'Restore Down' after a Maximize, does it get smaller or bigger?

Can you resize the window by moving the edges?

Try going to the screen resolution page and change it to something else and then reject the changes (it will go thru all the reset process for the video card).

Another thought: are you using the default theme? If you have something else, try the standard/default theme.

Second question first: I'm pretty sure that I am using the default theme - it is the only one there when I look under Add-on Manager > Appearance.

Then it gets even stranger. Sometimes the window that opens can be resized, other times not! I can reproduce the following two cases:

1. If I double-click on the email in the thread pane, then the opened message window is about 40% as wide as the monitor screen, and about 150% as high. It cannot be maximised or resized. This is the problem that triggered this support request.

2. f I open the message window by scrolling to the bottom of the message in the message pane and clicking on the 'in a new window' button at the bottom left, then the opened message window is resizable. Not only that, but TB remembers the opened window size and location for subsequent openings. This is the desired behaviour!

The funny thing is, I'm sure that I have had the message window open in the dimensions described in 1, above, but that I can then resize it (Alt-Space x). I cannot now reproduce this ability to resize the window when it opens at those dimensions.

Thanks so much for persisting with this.

Glad to be of help; I always like a good mystery ;)

Are you satisfied with the situation now?

You might look at the Windows themes to see if that makes any difference.

If you would like for me to look at it anymore, go to Help -> Troubleshooting Information -> Send via email to barry@barryduggan.info

Good luck, Barry Duggan (duggabe)

Thanks Barry, I'll take up your offer.

This is to document what was discussed via email.

1. When a message window is open, View -> Zoom -> Reset (or Ctrl-0 that's a zero)
2. Try View -> Threads -> Ignore Threads.
3. Do you have any anti-virus software running?
4. In Windows Control Panel -> Ease of Access, have you changed any screen settings?
5. Try running without add-ons. Help -> Restart with add-ons disabled.

OK, the culprit appears to be Thunderbird Conversations 2.8.1 With every other add-on enabled, but this disabled, it works as expected. With this enabled and every other add-on disabled, we get the problem. I've had a quick look at the reviews for Thunderbird Conversations, and there is no mention of this problem, though there are quite a number of complaints about other issues. There are >77k users.

Thanks so much Barry for your help!