Rendering atlassain.net pages

Since version 80.x Firefox Portable is not able to display some atlassain.net pages, e.g. login page to https://teleconnect.atlassian.net (no public page, but login screen should appear).
It regards to newer pages of atlassain.net, older pages seem to work correctly.
The Firefox 80.x installed version (Win10) -not the portable one- does not have this problem.
Any idea?
Thank you
Andreas