Prevent sidebar close/changes
The Firefox Sidebar is a very nice feature that I use to provide favorite bookmarks to an non-computer-savvy elderly parent. Unfortunately many times the "X" in the upper right of the sidebar is accidentally clicked which results in a phone call or remote login session to recover. It would be helpful if there was an option to "lock" the sidebar so it cannot be closed by an unsavvy user. In addition, it would be valuable if the sidebar lock prevented the user from collapsing/expanding (changing) the bookmark lists in the sidebar. Once they are "locked" (by a computer proficient person), no changes are allowed to the sidebar by a careless user.
Alla svar (2)
It may be possible to do that with userChrome.css (possibly by hiding the X) but that seems like a bad idea unless there is some other way of closing the sidebar. The sidebar can be reopened with the keyboard shortcut ctl+b.
I can't think of a way to prevent closing the sidebar. Did you set up the optional Sidebars toolbar button to open/close the sidebar?
It's pictured in the support article:
You might need to use the Customize screen to add it.