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I have the same problem, In firefox, the calendar shows at the lower part of the page while it works perfectly with ie. I just observe that when i remove the http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">, it works ok. I tried to download the attached file also, but it didn't work for me :(

The issue can't be reconstructed locally. With latest codebase calendar renders correctly. 
If issue still occurs for you - please provide any kind of sample or demo link where problem can be reconstructed ( you can send it directly to support@dhtmlx.com )