Click the lower right corner and that will drag the box into bigger size.

Your suggestion solved the original question, but brought an additional problem. How can the column size be changed?

As mentioned by david: The width of the columns displayed is related to the width of the 'Passwords' window and the width of the 'Passwords' window relates to the width of the Thunderbird window. Both of which can be expanded as described.

Columns cannot be altered manually, but you can hover over an item in the 'Provider' column and a tooltip appears showing all the provider information. Alternatively, as a temporary option you could disable the 'Last changed' column to offer more space for the other columns. Click on the far right icon in header area and uncheck any option selected. You can enable it again afterwards.

Another option: Right click on a line and select to copy desired element. Then paste it into a simple text document like Notepad, so you can read at leisure.