Disappearing Commands in Mail Toolbar

[JamesJoey]

When I customize the Mail Toolbar and attempt to drag a command onto it,
the command doesn't get dropped and it disappears from the 'Customize Toolbar'
screen.
I'm unable to retry because the command is gone.
I've tried to 'Restore Default Set' but that doesn't work.

Any help will be appreciated,
James

[JamesJoey]

I even uninstalled Thunderbird and that didn't help.

What's with this program??

[JamesJoey]

I had to 'Restart with Add-Ons Disabled.

That brought back the missing commands.

[DanRaisch]

I even uninstalled Thunderbird and that didn't help.

You're posting in a forum dedicated to SeaMonkey issues but referring to Thunderbird. Which is it that you are running?
When things work properly in Safe Mode but not in Standard Mode it often indicates that an installed extension is causing problems.