The answer in short to spare you any misleading in this case:
It's three known bugs: In sqlite, and/or in Firefox's handling of sqlite, and in the operating system Win 7. (Win 7 shouldn't allow a program to stall the whole system.) Deleting the faultily huge "places.sqlite-wal" in the Firefox-user's profile folder solved it.
My notebook system is: Win 7 Home Premium 64 bit. One classical harddisc and one SSD are in the notebook. The SSD is the boot disc. Intel processor with two cores with two threads each. (4 threads together.)
Right now seconds but mostly 5 to 20 minutes after startup my system stalls.
When the system stalls, then the task manager (if still accessible) shows that "NT Kernel & System" (always with PID 4) goes up to 25 and 26 % cpu usage. The program window I was working with when the stalling started is partly responsive, sometimes not responsive at all, sometimes when clicking on the window it then gets covered with white half opaque.
Sometimes others program windows are still full accessible. Sometimes ctrl+alt+del takes me to the win user logof/change/ etc page, but though there sometimes clicking is possible - it never does work whatever I select, if anything it takes me back to the desktop and program windows.
After start of the stalling it takes seconds up to 2 minutes and then nothing of the system is accessible anymore.
Only holding the on/off button for seven seconds works anymore to turn the notebook of by stopping the power.
I tried process explorer already, with process priority setting to maximum. But once the stalling appears, I can't save anything from there anymore. (Could I make a continually saving log with process explorer?) I also tried WhySoSlow beta, but nearly to the same avail. I didn't try to record everything (processes) with WhySoSlow so far, because I have bad experiences with full steadily running logs. (But I will record everything if neccessary.)
My question: What should I try now?
I'm really out of ideas, as even Process explorer on highest process priority stalls.
My guess is that the Win 7 system has acquired a malfunction (or changed setting) so it stalls when the HDD is going to sleep or is woken up again. Running music from the HDD in an endless loop I had no stall for hours. But when writing the last sentence of the first version of the long text I already had written for here, it stalled again. ): Maybe the whole mp4-music file ended in cache and so Win 7 again tried to put the HDD to sleep? (Or wake it up later on.)
The system worked fine for at least over 6 months. (Actually I believe I put the SSD over two years ago into the computer.)
When writing a necessary longer letter around 28 December 2015 the system started with stalling. Beside other things I tried: I exchanged the classical harddisc by another hdd. And put the two discs (new HDD and SSD) in another technically identical notebook. The stalling happened there, too.
So I already narrowed the cause: 1. It's either the SSD. or 2. My Windows 7 has acquired some problem it didn't have before. (I can't exclude that I changed some energy settings, to make the Notebook more silent, for example by letting the HDD going sooner to sleep.)
I now tried to run an mp4-file (located on the HDD) in an endless loop, and the system is running now for hours!
So I assume, that it has something to do with how Win 7 handles the HDD or the SSD.
--- Oh no! Just when I was writing the last sentence the system stalled again after hours. ): I changed the wording of the question, and now ask in general.
(P.S. I have a 64 kib/sec internet connection currently.)
MEDION P7624, Microsoft Windows 7 x64 Home Premium, Intel(R) Core(TM) i3-2350M CPU @ 2.30GHz, 4.0 GiB expandable to 32.0 GiB