Steps to reproduce:
1) Use Firefox 43.0.4 on Windows 10. We have not been able to reproduce this in any other combination of browsers or versions, including Firefox on Windows 7.
2) Open a page in the visual editor. (Therefore, you are editing the whole page, **not** a section.)
3) Make a change. Switch to the wikitext editor. Choose "Keep changes".
4) Make a change. Switch to the visual editor. Choose "Keep changes".
5) Make a change. Switch to the wikitext editor. Choose "Keep changes".
6) Discover that all of your changes were lost – this time, but not any of the previous times.
This has been reported by one editor, @3family6. For this editor, it is consistently reproducible in 100% of attempts by following this exact process, both while logged in and while logged out/using a private browsing window.