When writing new email, browser opens randomly?

[browseropening]

A friend of mine uses Mozilla Thunderbird and whenever he starts writing an email, it will bring up the default browser (Firefox in this case). The browser that comes up is the default one. This began occurring a week ago and never happened prior. The default was changed from Firefox to Chrome, and it stopped bringing the browser up for a day before resuming the same thing, but with Chrome this time. The page that is brought up is just a home page so there is no website/link it is visiting. We ran CCleaner and Malwarebytes with no bugs or viruses appearing. I used a remote desktop application to access his screen and write an email for myself, to test that the keyboard isn't the issue, and the same thing happened with me typing. There doesn't seem to be a specific key that triggers it. No keys other than the letters and numbers were touched. There was no touch pad that could ruin inputs. You can type for a bit before it comes up, then it'll come up randomly after as well. There does not seem to be a pattern. The OS is Windows 10 Pro. It isn't doing any harm but it becomes extremely annoying when you try to focus on composing an email. Does anyone know of things similar or ways to try to fix it from randomly opening the browser when typing? I use Thunderbird myself and I never had this issue but my friend just recently got it. Any advice or help would be greatly appreciated!

[tanstaafl]

Just to be clear, this happens when they press the Write button from within Thunderbird?

Does the same problem occur when replying?
Does the problem occur if they create a plain text message rather than a HTML message?
Does the problem go away if they use safe mode (help -> restart with add-ons disabled) in Thunderbird?

What version of Thunderbird are they using? See help -> troubleshooting information for that info.
Did this problem occur after a recent upgrade or has this been a long term problem?