Your home page setting shouldn't change on its own. Does it work to change it back? I suggest using a full URL:

https://www.google.com/

here:

Startup, home page, tabs, and download settings

If your home page is .com and it gets redirected to .uk, that's Google helping you by showing you what it thinks is local to you. If you visit this address, Google will set a "no country redirect" cookie:

https://www.google.com/ncr

and stop redirecting for as long as the cookie lasts.

Does that help?

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.