Problem with ajax HTML Editor in Firefox

Hi Everyone,

I am using Ajax control tool kit 4.0 with my application.

Am opening a popup which contains an iframe with ajax Html editor. 

While opening the iframe popup, the ajax html editor not getting loaded properly in FireFox.

throws TypeError: a.ownerDocument.defaultView.getComputedStyle(...) is null

Can any one help having ideas about this issue with mozilla. Please suggest solutions.

Note: I cannot update the Ajax control toolkit version.

Thanks in Advance