The solution:
There are occurences of this reported for over 3 years on the mozilla-bug website.
It's Firefox stalling the system when accessing too often or with too big data sizes the file "places.sqlite-wal" in the Firefox-user's profile directory. Related or as cause of this that file becomes very large. My "places.sqlite-wal" became over 1.8 GByte.
After deleting the too huge "places.sqlite-wal"-file no more stalling occured.
[ You might increase the sessionstore-time in about:config, but I do not know if this advice I read somewhere really is a solution. At least I can't see why making a session store every 15 seconds default (value 15000 ms) is necessary.]
In the internet I found examples with Firefox running on Win 7 Home Premium and using an SSD or an classical HDD. The biggest file size reported I found was 6.5 GByte.
I class this as three serious bugs:
- Firefox seems to keep allegedly deleted info in the "places.sqlite-wal" file, as sqlite itself or Firefox's handling of it seems to be faulty. A BIG SECURITY AND PRIVACY BREACH!
- Win 7 should definitly stop Firefox from being able to make the whole system 100% unresponsible for a long time. I found cases from 90 seconds to cases where the system still didn't respond after 10 minutes.
- The sqlite-website itself reports about errors happening when writing too often or too much to or in a too large wal-file.
I go even further and say that Firefox's switch to the use of the sqlite-database was fishy and it's handling of bugs related to this sqlite is fishy: Mozilla's staff comments on that topic were not helpful, to put it mildly. The problem is called as seldom.
In my case it was only possible by chance to find the reason, as ALL process monitoring programs I tried were not able to record what was happening - even Process Explorer with thread priority set to the maximum didn't respond anymore. (I wasn't able to download Microsoft's XPerf suite due to my slow internet connection with 64 kbit/sec.) People will in the end just make a new installation of Firefox or even the OS instead of reporting the bug.
My own conclusions after reading those things I found was that the sqlite use allows spying on the user.
Idependently when writing about the big file size someone else told me that the places-files are the most yielding ones in computer forensic.
P.S. How I found it? By chance. As last resort I was about to prepare cloning my boot partition to another harddisk. Because I only had about 35 GByte on the other harddisk I tried to delete huge files from my boot partition. Having an SSD it was quite riskless to run a search in Windows Explorer over the whole c-partition for the biggest files, and "places.sqlite-wal" was the biggest. (I already removed win7's and x-systems' installation files and 3 movies.) "places.sqlite-wal" was by far the largest file then. After deleting it, the stalling never occured again.
All the other things I did were lost time.
Some links: https://bugzilla.mozilla.org/show_bug.cgi?id=871908 And explicitly about file system problems somewhere in http://www.sqlite.org/draft/wal.html