Your picture isn't very helpful since it doesn't deliver any information! Start Thunderbird in "troubleshoot mode" by holding the Shift key while launching TB. This will open a dialogue box (picture): check "Reset toolbars and controls", then click on "Make Changes and Restart"

I agree, the screen grab is not very useful - that's because having all the windows open zero height isn't very useful either. Here's another screen grab with the zero height message window highlighted

I've tried creating a new profile, removed and reinstalled Thunderbird all to no effect.

Given that every Thunderbird window I've opened is zero height (unless previously maximised), including running the profile manager (thunderbird -p) it points to a problem below the level of configuration. Wherever these default window sizes are saved, the program is'nt using them.

Although a separate problem I've noticed that I can install a Thunderbird dictionary language pack addon (en-GB) but it's not listed as a dictionary I can choose.

Should have said, the versions are: Chromebook 139.07258.164 64 bit Thunderbird 128.14.0esr 64 bit

Here's another screen grab with the zero height message window highlighted

If this is a new compose window it is very small indeed!

I've tried creating a new profile, removed and reinstalled Thunderbird all to no effect.

Did you really create a new profile or did you just uninstall and reinstall Thunderbird?

Wherever these default window sizes are saved, the program is'nt using them.

Sizes (height & width) of Thunderbird windows (main, compose, panes, ...) are saved in the xulstore.json file in the profile folder. Did you try troubleshoot mode and reset toolbars and controls as I suggested in my first reply?

Although a separate problem I've noticed that I can install a Thunderbird dictionary language pack addon (en-GB) but it's not listed as a dictionary I can choose.

Dictionaries and language packs are two different things. Dictionaries are used for spelling control, they are listed in Add-ons < Dictionaries.... https://services.addons.thunderbird.net/En-gb/thunderbird/language-tools/ look in the "Dictionary" column Language packs are listed in Add-ons > Languages... and are used the change the UI language, e.g from English to French. Look under settings > General > Language & Fonts > Language > Set alternatives ...

Many thanks for your speedy reply. To answer your questions:

Yes that is a new compose window. As I also noted, all new windows: help, discard new email and even the profile manager all open zero height.

I moved the existing profile to another directory, created a new profile with the profile manager then opened Thunderbird. Then with that new profile in place removed and reinstalled Thunderbird. Finally moved the original profile back into the .thunderbird folder and used the profile manager to switch back to it. No change any point.

Following other advice I have previously deleted xulstore.json - thunderbird recreates it but new windows are still zero height

I have tried troubleshoot mode and reset toolbars and controls as you suggested - no effect

Dictionaries and language packs are two different things. - a fair point. I had confused them and have now installed the en-GB dictionary addon and can select it.

I moved the existing profile to another directory, created a new profile with the profile manager then opened Thunderbird. Then with that new profile in place removed and reinstalled Thunderbird. Finally moved the original profile back into the .thunderbird folder and used the profile manager to switch back to it. No change any point.

Yours seems rather an odd way to test TB in a new profile. After creating your new Thunderbird profile - without importing anything from your old profile - did you set up an email account and test opening a new compose window ("New Message" window)?

By the time I did this I had found that opening Help shows the same problem. Not to mention thunderbird -p

I'd like to fix all these zero height windows - my assumption is they all have a common underlying cause. This may not be true but I hope Occam's razor applies in this case