You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

FredMcD said

You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder. Type about:support<enter> in the address bar. Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox. Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory. Locate the above file. Then rename or delete it. Restart Firefox. Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

there are no sessionstore* files in this folder, only previous* and recovery*

Update/solution: There is a user.js file in my profile's folder. I did not create it. The firefox documentation states that it doesnt exist by default, it has to be created. http://kb.mozillazine.org/User.js_file

Content of the user.js file:

   user_pref("extensions.blocklist.enabled",false);
   user_pref("extensions.blocklist.interval",31536000);
   user_pref("extensions.blocklist.url","");
   user_pref("extensions.blocklist.itemURL","");
   user_pref("extensions.blocklist.detailsURL","");
   user_pref("security.mixed_content.block_active_content",false);
   user_pref("security.mixed_content.block_display_content",false);
   user_pref("app.update.staging.enabled",true);
   user_pref("browser.safebrowsing.appRepURL","");
   user_pref("app.update.silent",true);
   user_pref("browser.safebrowsing.updateURL","");
   user_pref("browser.safebrowsing.reportURL","");
   user_pref("browser.safebrowsing.reportMalwareURL","");
   user_pref("browser.safebrowsing.reportMalwareErrorURL","");
   user_pref("browser.safebrowsing.malware.reportURL","");
   user_pref("services.sync.prefs.sync.browser.safebrowsing.enabled",false);
   user_pref("services.sync.prefs.sync.browser.safebrowsing.malware.enabled",false);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);
   user_pref("browser.sessionstore.resume_session_once", true);

and there is the problem. After deleting all the "user_pref("browser.sessionstore.resume_session_once", true);" lines but one and setting the remaining one to false my problem was solved. I guess some extension or plugin created this file and since it overrides any other change it didn't matter what setting I would change in the browser itself.

In the end, I set the user.js file to read-only to prevent this from happening again.

That was very good work. Well done.

If you didn't create the user.js file yourself then you should remove this file. It is likely added by other software without informing you.