In your More System Details on right of your post your User Agent appears to be modified with ql66u54b on end.

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 ql66u54b

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

yes, that's right: dom.push.userAgentID;c0bc4eeaaddf430b9be323fc3201b2ae general.useragent.override;Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 ql66u54b

This resolves the problem temporarily, if I restart firefox, the user agent is modified back.

Disabled all the addons, started firefox in safe mode, - the same: general.useragent.override;Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 ql66u54b

MihailS, go to about:congif (enter as url and press Enter), when you will see setting, type in search input "ql66u54b". If was found, delete this from string, if not, go to nex step.

Download and Install Total Commander. Find all folders wichi contains "Mozilla" or "Firefox" and save paths to it. Next: in Total Commander and in File Search start to search for each path, type in With Text input (don't forget to check) "ql66u54b"

MihailS said

yes, that's right: dom.push.userAgentID;c0bc4eeaaddf430b9be323fc3201b2ae general.useragent.override;Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 ql66u54b This resolves the problem temporarily, if I restart firefox, the user agent is modified back.

Either an extension is adding this or you have a user.js file that is overwriting the changes made to prefs.js file as you make change in about:config

http://kb.mozillazine.org/user.js_file

in \Mozilla Firefox\defaults\pref found local-settings.js

pref("general.config.filename", "filename.cfg");

this filename.cfg have overridden pref("general.useragent.override", ....)

config file and script pushed on my PC by domain policies. Now resolved.

Thank you for help.