hello Claude R,

Type about:preferences#advanced<enter> in the address bar.

Under Advanced, Select Update. Check the settings.

for more understanding I attached screenshot,

tell me when resolved your problem,

Thank you!

How do you completely turn off update checks in Firefox v64?
You can use a policies.json file to disable updates for all users.
https://github.com/mozilla/policy-templates/blob/master/README.md
or
https://winaero.com/blog/disable-updates-firefox-63-above/

how can i stop the continuous pop up for upgrading to new version?

Don't! If you are that unhappy with Firefox, just switch to a different web browser. Or give up on doing the type of user modifications that you have been doing. With you still using Fx75 I an "reading in" to issues that MANY other users have reported with userChrome mods that worked with 75 and are majorly broken again with 77, and where 76 they still worked.