see this answer https://support.mozilla.org/en-US/questions/1217097#answer-1110106

Thank you for your replies, however none of the suggestions worked. The problem still exists.

ok,

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!

Thank you.

Thank you again for a reply. I had already tried the Refresh procedure and It did not solve the problem.

That sounds like a corrupted Browser. It shouldn't move unless something in the program to save the last open location doesn't set then it will move in random ways.

rename the xulstore.json

if it did'nt help just Type about:support in the address bar and press enter.

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

Delete or rename the xulstore.json file. and Restart Firefox.

hi, does it make a difference if you enter about:config into the firefox address bar (confirm the info message in case it shows up), search for the preference named browser.startup.blankWindow & double-click it and change its value to false?

Thank you again for all of your replies. Had tried all the previous suggestions but to no success. The info from philipp has cured the left going creep in window size that I was experiencing. Again thank you all for your contributions to help resolve this annoying situation.