about:blank is usual one used.

From the Menu Bar --> View --> Sidebar Untick any thing that is ticked.

Please let us know if this solved your issue or if need further assistance.

My mistake. about:blank is the one I use. Unchecking just closes it temporary, like clicking on X whatever I do the sidebar always open at startup I don't want a sidebar. almost never use it

Is there a setting in config?

any answers?

I am not the hacking about:config person. I need to search for all my answers. As all previous ways to do this are now of no use in 57+ I have no idea unless missed one that works .

To me it seems it was a bad update and it is a file issue that should be correctable by doing : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

And if not TEST in Safe Mode : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

and Make a New Profile and TEST : https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Something will cure this irritation.

Please let us know if this solved your issue or if need further assistance.

You can rename/remove xulstore.json and if present delete localstore.rdf in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

thank you both.

Unfortunately the sidebar reappear IF you once closes with a sidebar open. The solution is to write-protect the new Xylstore.json right after it is restored.

Keep in mind that the sidebar and toolbar state is not the only data that is stored in xulstore.json. It also stores sizes and positions of various windows.

Yes, when you close Firefox with the sidebar open then the sidebar opens automatically on the next start and you will have to close that sidebar, either before you close Firefox or after the start.

The reason I writeprotect is that whether or not I close without the sidebar it will open with the sidebar enabled. Once I use the sidebar Its always there. I can change what is shown but thats all.