Yesterday my computer became very slow, so I restarted it (included running chkdsk).
Afterwards it behaved as normal, and I put it into sleeping mode as every evening.
When turning on the omputer today, it started from scratch, not from sleeping modus.
When starting Mozilla, it opened with only one window and one page, Mozillas start page.
It has history of which pages I've visited, but is not able to restart my previous session.
Visiting the event log, I find nothing suspicious, only about Flash player which I updated without restarting.
Therefor I wonder if Adobe might have destryed my day.
Flash player has no APIs in it which would affect Windows sleep settings or Firefox Session restore settings. It only affects Flash content (data storage and protected content access) on pages you view, and webcam settings (cam and mic) if you use them.
To use an analogy, it's like buying a new Blu-Ray player (Blu-ray only, no tuner or anything else) and after connecting it, you discover that all your channel settings on your TV have been reset to factory defaults. There's nothing in the player that could have caused it.