Ctrl+Z does not work as expected in this forum. I had many bad surprises with that (post completely erased or a big part of it). To replicate the issue, you just have to type and use Ctrl+Z. This is not related to the browser.
Strange... I have no problem this morning with CTRL+Z. However, many times in the past, CTRL+Z erased my whole post or a big part of it (whatever browser I may use: IE/Firefox/Chrome) instead of cancelling the latest changes. I also discovered this morning that you can cancel the CTRL+Z with CTRL+Y so you can close this case.