If starting `web-ext run` opens Firefox with an update pending, then `web-ext` won't quit after closing Firefox

[Juraj-Masiar]

This is a common scenario with Firefox Nightly - because it updates every day. For example:

1. you start Firefox Nightly to test something, it will download update in the background and prepares it for install (you can see in the "About Firefox" that the update is ready and you need to restart Firefox to apply it)
2. close Firefox Nightly - don't restart it!
3. start web-ext with: web-ext run --firefox=nightly --verbose
4. close Firefox Nightly

But the web-ext doesn't exit, it will hang with the last log being:

[C:\Users\juraj\AppData\Roaming\npm\node_modules\web-ext\lib\firefox\remote.js][debug] Received "end" from Firefox client