https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt xulstore.json file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/Profiles
• Linux: *~/.mozilla/firefox/Profiles
• Windows: %AppData%\Mozilla\Firefox\Profiles
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles
• Windows XP: C:\Documents and Settings\<user>\Application Data

\Mozilla\Firefox\Profiles\

Search for the above file. Then rename or delete it. Restart Firefox.

Please explain how to open my file browser to the hidden location you mentioned - for Windows. Also, does it make a difference if I "rename or delete" the file you mentioned? Thank you

Please explain how to open my file browser to the hidden location you mentioned

To open File Explorer, select Windows logo key + E or go to Start and type File Explorer in the search box. Select File Explorer in the results.

In the address box, enter; %AppData%\Mozilla\Firefox\Profiles

Also, does it make a difference if I "rename or delete" the file you mentioned?

If a file is corrupted, sometimes the best thing to do is to remove it. As to Firefox, any missing files will be recreated as needed.

Thanks for your reply. I located the \Mozilla\Firefox\Profiles folder but I did not delete it. I noticed that the Profiles folder had a lot of my data, including bookmarks. So I want to ask - once you are in that file, isn't there a way to fix the problem I have without deleting the whole file? Otherwise, what about deleting the Firefox program and starting over again?

Delete only the xulstore.json file as I directed here; https://support.mozilla.org/en-US/questions/1355720#answer-1454619

This will not harm other files. The xulstore.json file only takes care of the window size and the toolbar. This should fix the problem with the size of the window.

Thank you ---The Firefox page opens correctly now!!

Glad to help. Safe Surfing.