WebPlus X5: text frame looks different with different browsers [WPx5]

I have always wondered why text frame is displayed in different ways using different browsers...
Logically thinking the same size of the text frame, and the same length of the text... - it should be displayed in the same way using any browser. Right?
But this is how it really is: I noticed that IE, Opera and Google Chrome displays it as it should be and as I want it to be displayed... but Mozilla Firefox it in a different way. Mozilla adds scroll bars at the right side. Actually they do not appear in IE, Opera and Chrome. So it seems like in Mozilla the same text doesnt fit in the same text frame just as it does fit in IE, Opera and Chrome.

If there is any chance to avoid this?

By right clicking on the text frame > text format > overflowing text > you can choose from 4 different options... like hidden, overflow, auto scroll bars and fixed scroll bars. If I choose anything else but auto scroll bars or fixed scroll bars - the text is not shown or just overflows the text frame in Mozilla. While in IE, Opera and Chrome it perfectly fits the text frame.

For example: http://thendestories.com/NDE_story0017.html

or

http://thendestories.com/NDE_story0001.html