I’m proud to finally have achieved to find a way to reproduce a bug which drove me crazy many times! ^^

1. Using Firefox, edit with visual editor a page which contains a colon following a space like this: words : other words (you may use random article on fr.wp, they almost all contains :)
2. Put your cursor after the space.
3. Press backspace key on your keyboard until the space is removed (only once if you put your cursor between the space and the colon).
4. Type anything (except Carriage Return)
5. Try to save: what you typed in previous step is not saved. (You can only Review your changes in wikitext, the text doesn’t appear too).

This bug doesn’t appear with Chromium.
It doesn’t appear with other characters than colon (I have tried ? and ! which both generate   before them as : do.)
It doesn’t appear if you remove the space using Del key.