>>For a while, I get blank page (URL https://www.levelextreme.com/ViewPageGenericWorker.aspx) after the first login in Firefox. Subsequent login works.
>
>I cannot simulate this. I have Firefox 43.0.4.
>
>Can you give me step by step a simulation pattern?

I have Firefox 43.0.4 but the problem existed in few previous version as well.
Firefox is set up to remember all open tabs on browser closing.

After I launch Firefox next day (after cookies expire) and switch to UT tab I get prompt for login and blank page after that.