This is not an issue with Firefox. This is working fine on Firefox when I test it. I have no issue with the sidebar not automatically appearing in new windows, even after I've closed Firefox.

The issue is likely specific to your computer then. This is probably an issue with a setting or something along those lines. If you run Firefox in Safe Mode, do you still have issues with the bookmark sidebar not carrying over to new windows?

Thanks for your response. Safe mode doesn't change anything.

The issue only appeared after the automatic upgrade to v65. I'll try to be more specific. The issue does not occur when I open a new window by left-clicking on a bookmark. It only happens when I open a new instance of Firefox from the icon I have pinned to the taskbar. In that case, the "Bookmarks" option in the Firefox View's Sidebar menu is unchecked, even if I click on one of several instances of Firefox that are open on the task bar with the Bookmarks sidebar enabled and the option checked. I also just found that if I click on Firefox in Windows Program menu, I also don't get a Bookmarks sidebar. I have no idea how to change that default...

--- Huh. It must have been the computer. I closed all instances of Firefox and then when clicked, it came up with the sidebar. *Slaps head*

Firefox stores the sidebar setting in xulstore.json in the profile folder. This file is probably only written when you close all Firefox windows and the last window that is closed will set the sidebar behavior.