Tested.  This does ONLY happen in Firefox.  I believe this is due to the way Firefox interacts with the system clipboard.  I have foud that this only happens when you copy INDENTED code.  If you unindent the code and paste it in the form of:

 

<p>

text

</p>

 

rather than:

 

<p>

     text

</p>

 

the problem goes away.  I don't know how easy this is in other editors, but in Dreamweaver, you can simply select all the text, hit SHIFT+TAB a few times, and it removes all the indentation.

 

Hope this helps.

Max