Steps to reproduce:

1. Load a page in Firefox or open a message in Thunderbird.

Actual behavior:

An error is displayed that can break say all on page load.
ERROR - queueHandler.flushQueue (06:11:25.625) - MainThread (6504):
Error in func VirtualBuffer.loadBufferDone
Traceback (most recent call last):
File "queueHandler.pyc", line 55, in flushQueue
File "virtualBuffers_init.pyc", line 470, in _loadBufferDone
File "browseMode.pyc", line 1280, in event_treeInterceptor_gainFocus
File "enum.pyc", line 354, in getattr
AttributeError: Focus

Expected behavior:

No error.

System configuration

NVDA installed/portable/running from source:

Installed

NVDA version:

alpha-21672,856adec6

Windows version:

Windows 10, Version 10.0.21296 Build 21296.

Name and version of other software in use when reproducing the issue:

Firefox 87.0a1, Thunderbird 78.7.0.

Other information about your system:

N/a.

Other questions

Does the issue still occur after restarting your computer?

Yes.

Have you tried any other versions of NVDA? If so, please report their behaviors.

Yes, alpha-21668,e8f14f9a works normally.

If addons are disabled, is your problem still occuring?

Yes.

Did you try to run the COM registry fixing tool in NVDA menu / tools?

Yes.