Spacing/rendering problems are generally cross-browser issues related to the absolute positioning that Marketo's landing page WYSIWYG editor employs.
I have run into similar cross-browser spacing problems between Chrome and Firefox. If you have someone code-savvy on hand, our fix was to apply some browser-specific CSS to our landing page templates, to help correct the rendering discrepancies.
Here's a StackOverflow thread that steps through how to do this for different browsers, including IE and Firefox:
http://stackoverflow.com/questions/952861/targeting-only-firefox-with-css