Confirmed. However, This is likely to require updates to the virtual buffer system, which is probably not going to happen.

@challsworth2, @LeonarddeR, can you still reproduce this issue?

Testing with NVDA 2019.1.1 and Thunderbird 60.6.1, this issue is still reproducible. It seems you can select a i.e. link in browse mode with shift+arrow keys which is reported by NVDA. However, the context menu looks differently if you select the same link while you are in focus mode. In focus mode, NVDA does not report anything if you press shift+arrow keys to select the link. But When you press context menu, there is an entry called "bing search for ".

In conclusion, it seems the behabior when selecting a link in browse mode seems to be separated from the behavior when selecting the same link in focus mode.

This is no longer a feature in Thunderbird 68.7, at least not from the context menu. Both context menus in browse and focus mode are now identical. Closing as invalid.