could you please provide further information on:

1. What is your Thingsboard version (like version number, Community/Professional Edition)?
2. What does your dashboard look like (how many entries/entities, does the table have/how many widgets are in the dashboard)? Do you use any cell/content-style functions?
3. Maybe you can share a screenshot of the missing & working scroll bar (and since you can see the popup, can you scroll in that popup with the mouse even though you don't see a scroll bar?). (black out private information before sharing)

I want to hide those at first and then let the user select the figures to display by using "Columns To Display"

This is a simple setting you can turn on by going into your table widget, in the Data-Tab below your Device-Source you should see the Columns. Click on the Gear-Icon of a columns and change those Settings:

Default column visibility: Hidden
Column selection in 'Columns to Display': Enabled

I was not able to reproduce your problem, but in my test, the scroll bar style was different between Firefox and Edge (styles can often be different between browsers).

Image of the Difference (on demo system v.3.8.1.9)