Can't Allow Permissions to your Extension/App

[sus007]

Firefox Version : Nightly 71.0a1 (2019-09-17) (64-bit)
OS : Windows 10 Pro x64

Clicking the "Allow" button simple redirects to the "Unable to Connect" White Blank Screen, hence can't get your extension to work.

[flytaly]

Maybe it is too late and you are not interested in this extension anymore. But it could help somebody else anyway.

Recently I found out that Firefox Multi-Account Containers or Temporary Containers could break OAuth2 authorization.

Basically, Firefox Containers or Temporary Containers in automatic mode could close the authentication window and reopen URL in a new container/tab which breaks authentication. Maybe this is what happened in your case.

So the easiest way to solve the problem is to temporarily disable these extensions in about:addons page, authenticate and then enable them again.