span tag with width style attribute doesn't work in Firefox

I have the following code.
<span style="width:100px;background-color:Red;">text</span>
<span style="width:100px;background-color:Green;">test</span>
<span style="width:100px;background-color:Blue;">test</span>

In IE, it displays just like I intended:  100 pixels of red color, 
followed by 100 pixels of green color, followed by 100 pixels of blue 
color all on the same line.

Firefox ignores the 'width' attribute.  It only displays the red color 
enough to display the word 'test'.  I can't use the DIV tag because it 
forces a line break.  I need all this stuff to be on the same line.

What can I do to replicate the behaviour above in Firefox?

Thanks.