What version of Thunderbird, and which locale?
What language to be spellchecked?

I was on Thunderbird 68.8.1 but updated before posting my help request to 68.9.0. Problem still arises. Locale is UK. Language to be spell checked is UK English. Previously Thunderbird was on default US English, but before posting my help request i installed UK English Dictionary (Marco Pinto). Again this did not cure problem.

i installed UK English Dictionary (Marco Pinto)

Where did you get this from?

Make sure to install dictionaries from https://addons.mozilla.org/en-US/firefox/language-tools/

I got it from https://addons.thunderbird.net/en-GB/thunderbird/addon/british-english-dictionary-2/

after following the instructions at https://support.mozilla.org/en-US/kb/how-install-and-use-another-language-dictionary-th

But anyway, my problem with spellcheck occurred BEFORE I installed the Bristish English dictionary.

Hoping you can help....

my problem with spellcheck occurred BEFORE I installed the Bristish English dictionary.

For your UK locale Thunderbird an Ukrainian dictionary is built-in. Are you saying the spell-check problem already occurred for Ukrainian language?

Try to start Thunderbird in safe mode.
https://support.mozilla.org/kb/safe-mode-thunderbird

Does the problem go away?

My recollection is that before I installed the British English dictionary, my Thunderbird had the US English dictionary installed and the speller was not working then either.

I do not understand why a version of Thunderbird used in England, UK should have a Ukrainian dictionary built-in.

I have tried using TB in safe mode. I am not sure if it allows me to send emails without the spell check working. However, I have gone into Tools/Options/Composition/Spelling on the toolbar and have found that if I untick both "Check spelling before sending" and "Enable spellcheck as you type" then I can send mis-spelt messages. But, if I tick one or both of these options then my mis-spelt message freezes as described before.

Further to my last reply I have checked TB in safe mode again. What happens is that the spellchecker dialogue box pops up, but it has no alternative words where they normally are and all the other boxes are blank. But when I close the pop up box it does enable me to send the message unchecked and with its errors.

I do not understand why a version of Thunderbird used in England, UK should have a Ukrainian dictionary built-in.

Because that is what you said your Locale is, i.e. which localized version of Thunderbird you're using:
uk-UA = Ukrainian
http://www.localeplanet.com/icu/uk-UA/index.html

en-GB = United Kingdom of Great Britain

So if you downloaded the en-GB version of Thunderbird, then you don't need to install the British English dictionary, because it's already built-in.
If it's installed, get rid of it.

Then make sure to choose the desired language to be used for spell-checking. Note, there's a default language for spell check in the main Thunderbird settings.
The language can be changed on the fly in a compose window if a dictionary for a different language than your Thunderbird localized version is installed.

I have discovered that if, in Options/ Options/Composition /Spelling, I untick "Check spelling before sending", and "Enable spell check as you type" and then avoid pressing the "Spelling" tab on the compose window, than hitting Send will get the email away without it freezing. However, if I tick either one of those two options then the email freezes as before.

I will just have to remember to untick the items above and keep away from the Spelling tab, and then edit spelling manually before sending.

Disappointed though that the root cause of the problem remains.

Very odd. I'm on Windows 10 64bit, using Thunderbird 68.9.0 (32bit), en-GB and I happen to have the Marco Pinto English British dictionary as you.

I have been testing various set ups but cannot reproduce the error of a blank spelling window or freeze issue.

Could you do a test and see if anything is shown in the error console or whether a crash report was sent. Please check you have all the following and reselect to ensure they get updated.

• Tools > Developer Tools > select 'Error console'
• Select all the filter options eg: Errors, Warnings, Logs etc, so they are highlighted
• Clear the error console by clicking on the bin icon located top left corner
• close error console window - top right X

• Options > Options >Composition > 'Spelling' tab
• Do not select for first test: 'Check spelling before sending'
• Select: 'Enable spell check as you type'
• Language: select the drop down and select 'English (United Kingdom)' - it will probably be the only option, but select anyway. Please tell us if there is more than one.

Make sure the Crash reporter is set up to work.

• Options> Options> Advanced > 'Data Choices' tab
• select checkbox for 'Enable Crash Reporter'

• Options> Options> Advanced > 'General' tab
• Language: Select 'English (United Kindom)'
• Alternatives: Select 'English (United Kindom)' and click on OK.

It will now ask you to Restart Thunderbird to apply settings.

• click on 'Apply and Restart' button.

Now create an email to send to your self with spelling errors. As you type they should get underlined. Click on 'Spelling'.

Please report on results.

• Include any info that now appears in the 'Error console'.

• Help > Troubleshooting Information
• if you see a Crash Report please post the ReportID which will start with a bp-

Thank you for your detailed and very easy-to-follow troubleshooting instructions.

With the spell check set up in accordance with your script I am pleased to say that my mis-spelt test message had the mis-spelt words underlined and that the spelling correction box popped up and facilitated corrections as it should do.

There was one item in the error console: uncaught exception: 2147746065 and at right hand side of the line: autosync.jsm:251:30 Below that and appearing a short time after the above info, it now it says "No chrome package registered for chrome://browser/skin/identity-icon.svg

And while I was typing this and after I mis-hit a key there appeared this:

help

NS_NOINTERFACE: Component returned failure code: ex80004002 (NS_NOINTERFACE) [nsISupports.QueryInterface], and at rhs of panel: stack-trace-collector.js:84

As for crash reports, there are none shown for the last three days. Opening up All Crash reports reveals that the last ones noted were in January and April 2018