Source editor data not presented my new text but it's available on broswer navigate-back-function

I typed text into the source editor. Pressing SAVE/VIEW showed me the result, but WITHOUT what I have just entered. But astonishing: When I press the browser navigate-back-button, the WYSIWIG shows me the text I've entered. So the text isn't lost but isn't shown! By contrast, if I only press edit again, data is lost.

Hint: I didn't clear the cache in the meantime. Phenomenon occurs every time, even if I enter only one letter. I can't check whether this happens on EVERY wiki page, but at least on those few I've explicitly tested.

xwiki version: XWiki Enterprise 3.5 (according to login screen)
Browser: Firefox 11.0 (portable app)
Source editor: Source-tab of wysiwyg-editor.