User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:77.0) Gecko/20100101 Firefox/77.0

Steps to reproduce:

I started Thunderbird in a profile in a secure container

Actual results:

A dialog message came up saying that there was an unresponsive script. This has happened every time I opened it for the first time after the container was opened. However once it had been started, if I closed TB and restarted there was no unresponsive script.

Expected results:

Thunderbird should have opened without the unresponsive script

What exactly are you using to start in a "secure container"?

And when exactly did this start?

I have an AutoIT program that uses the line $SecureThunderbirdPid = Run($ThunderbirdExe & " -profile " & $ScriptDrive & "Settings\Thunderbird") to start Thunderbird with the given profile. I only mentioned it because I found that after it had started once it didn't give the problem until the container had been closed.
The problem started when I updated to TB 78. I waited a few days to see if it would stop.
$ThunderbirdExe is the path of Thunderbird in my computer and $ScriptDrive is the Drive that the container has been opened on.

Given that you have multiple items starting at the same time, you are likely seeing high disk IO, and maybe even high CPU load.
You have tested that this happens if Thunderbird is started later, well after a container has started, correct?

I'm not finding secure containers capabilities with AutoIT, so what is a secure container in this context?

What line is that? (What version are you using?) https://searchfox.org/comm-central/rev/b0826913e4dd6d1913c9974fcc4dcf89a29a6f6b/mail/base/content/folderPane.js#1755 seems not an alternative

The secure container is opened with VeraCrypt and is definitely open before there is any possibility of starting TB - I only mentioned it because the "error" of the unresponsive script didn't seem to happen at other times. It also never happened before and is happening consistently now.
I'm using TB78.01b

Maybe duplicate of bug 1647251.

Or bug 1652279 if ...

musiquegraeme, are you still seeing the issue?

Yes. It happens at least twice, maybe three times at least once a day. It may be that the number of the warning changes - I think I've seen 1730. Does that change anything. I usually hit continue, not sure what stopping the script would do. It does work eventually.
By the way I don't have any custom folder colours...
Blessings

Performance can bite you even if you don't set custom color. Let's assume this is a regression of bug 1637668. bug 1652279 is the next attempt to resolve this.

Just started up with TB 79.0b2 and had a non responsive script twice the number was 1283...

Please check this candidate build this weekend and tell us if the situation improves.
https://archive.mozilla.org/pub/thunderbird/candidates/79.0b3-candidates/build1/

Should also be fixed in 78.1.0

This situation has been completely resolved for me.:) Well done.!!