I am not sure it is intended to be persistent, unless you use restore previous session rather than a home page. Maybe there is some preference that can be set. Make sure you exit from firefox using Firefox button -> exit

It used to be persistent in previous versions (don't know exactly when it changed). Of course I exit normally with [X] or Exit. Restoring a previous session is enabled and seems to work. But a new browser window (when there are already some open) again is without sidebar, even if all the other windows have the sidebar enabled.

Is that a pop-up window that has the sidebar missing?

If it only happens in one window then close that window or tear-off the tabs to a new window to see if that makes the sidebar persist.

It's not a pop-up window, just a normal browser after clicking on the FF-icon on the desktop or starting "firefox.exe" on the command line. No, closing and reopeing it doesn't help.

Can you tell me where this setting should be saved? Then I can check if it is not stored at all or if it is not active while opening an additional browser window.

If I have a browser with bookmarks sidebar and I do a "open link in new window" then the new browser window does have the bookmarks sidebar. It's only a completely new browser that has no sidebar.

Such data is stored in the file localstore.rdf in the profile folder.

• http://kb.mozillazine.org/localstore.rdf
• http://kb.mozillazine.org/Corrupt_localstore.rdf

OK, but what is the name of the setting? I have already tried the fixing as mentioned in the first post. The xml syntax looks well formed.

-> Tap ALT key or press F10 to temporarily show the Menu bar

• go to View Menu -> Sidebar -> select Bookmarks

Check and tell if its working.

Yes, this works, as written in my first post. That was never the problem but the persistence across browser windows if I open a new one.

Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

Doesn't help, after restarting FF and opening a new browser window the sidebar is gone again (not in the restored windows, only in the new one).

I think I finally got it to work. I have a second computer (same setup) where it works. So I compared the localstore.rdf files and found the problem.

 <RDF:Description RDF:about="chrome://browser/content/browser.xul#sidebar-box"
                  sidebarcommand="viewBookmarksSidebar"
                  width=""
                  src="chrome://browser/content/bookmarks/bookmarksPanel.xul" />

The sidebar was enabled but the width was empty so in effect disabled. Also closing a browser with the sidebar showing didn't change this value. After manually entering a value I can now open a new browser window with sidebar enabled.

Glad it works, congratulations on discovering the solution.