Your best bet is to do mozilla refresh: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Try to rename/remove <b>xulstore.json</b> in the Firefox profile folder with Firefox closed. You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You could install the extension [https://addons.mozilla.org/en-US/firefox/addon/maximize-all-windows/ Maximize All Windows (on Startup)]