What is your anti-virus software?

AVG. Windows 7.

Try to start Windows 7 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

This did not solve the problem. Additional details:

On the select file screen (for attachments), I can select any file in the pane and it will successfully add it as an attachment. But if I try to scroll down the pane or change directories, it locks up.

Can you confirm you did start Windows in safe mode?

In addition, also start Thunderbird in safe mode (now called troubleshooting mode). https://support.mozilla.org/kb/troubleshoot-mode-thunderbird

Running TB in safe mode partially solves the problem. It now works for most directories, but for certain directories, when I open the attach file pane, it locks up. Are there certain types of files TB doesn't like?

Running TB in safe mode partially solves the problem.

Are you running any kind of AVG extensions in Thunderbird?

What is the size of your Drafts folder?

Are there certain types of files TB doesn't like?

No, there aren't.

I am not running any extensions. My draft folder has 1 email in it.

It still isn't exactly clear what's causing the lockups in your case. In general, anti-virus software is often the culprit when it attempts to scan large TB mail files, which are then temporary blocked, which you may notice as lockups. These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Well, I'm pretty sure it's not the anti-virus software, as it won't even let me select a file or change directories. for an attachment. So AV is not involved at that point.

Well, I'm pretty sure it's not the anti-virus software, as it won't even let me select a file or change directories. for an attachment. So AV is not involved at that point.

Not so! You are navigating the file system using a system generated file picker dialog. Any anti virus worth it's CPU cycles will be monitoring all file system activity.

I dumped AVG way back, around 2005-10 because it was no longer doing the job, it had followed Norton into an alarmist product that just made things slow. So I really am not familiar with it now. But other than defender I am not familiar with any of the products anymore.

stamps1 said

I am not running any extensions. My draft folder has 1 email in it.

the number visible has nothing to do with the size of the folder on disk. Right click the folder select properties and then read the size. It might surprise you. Right clicking the folder and selecting compact allows the size on disk to shrink to only the current content.

Running TB in safe mode partially solves the problem. It now works for most directories, but for certain directories, when I open the attach file pane, it locks up. Are there certain types of files TB doesn't like?

What exactly does that mean? Are these directories on USB drive or other external location to your device? Are the directories stored outside of the "my documents", "my pictures" etc standard windows folders?