We've just pushed another update to the WYSIWYG editor. This will increase the stability of the new post editor under Mozilla/Firefox on the PC. Previously, longer posts would tend to crash those browsers in Compose mode.
As an additional note, we're completely committed to resolving the cross-browser issues with this new feature. Most of us here use something other than the latest version of IE and we want to make sure that everyone can take advantage of the enhanced editor. Thanks for letting us know about the problems you encounter and for your patience as we fix them.