Are you saying some history is removed but not all? What is being removed? One or more websites? No pattern?

Make sure you were not using Private Browsing.

Some programs can remove data from the browser profiles. Make sure to turn this off.

I did in fact not tell the most important fact: It deleted my older history, 6 months of the oldest recorded history was removed.

Firefox automatically calculates a maximum for the number of visited websites to keep, so it is possible that to exceed this maximum.

You can find this maximum if you click "Verify Integrity" on the "Help -> More Troubleshooting Information" (about:support) page.

• History can store a maximum of xxxx unique pages
• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

Thank you for this information.

How does Firefox calculate the maximum for the number of visited websites to keep and from which parameters is this maximum number dependant? Is it individual for every user, dependend of e.g. disk sice?

Is there a way to manually set this maximum to a different number, or to disable this maximum number resp. set it to infinity?

I really do not want to use any data of my history due to unpredictable automatisms.

See the comments in the source code:

• https://searchfox.org/mozilla-release/source/toolkit/components/places/PlacesExpiration.jsm

As you can see under nsPlacesExpiration() (line-425), you can possibly create a new Number pref to override the calculated maximum, but be cautious not to go too high as I'm not sure how many digits it can accept and also places.sqlite might grow too big.

• places.history.expiration.max_pages

What is the current size of your places.sqlite in the profile folder ?

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I found that the thing I want to achieve is set in the variables places.history.expiration.transient_current_max_pages and places.history.expiration.max_pages in about:config. Can you provide more information about that?

And is there a chance to recover deleted parts of my history from places, and if so, how?

places.history.expiration.transient_current_max_pages was a read-only pref that stored the maximum I posted above about. This pref is no longer supported as far as I know, but the places.history.expiration.max_pages pref is still honored when it exists (i.e. you created it as a number pref).

. . . is there a chance to recover deleted parts of my history from places, and if so, how?

The bookmarks are backed up automatically. Sorry to say, history is not.