This answer is rewritten entirely to explain the sidebar behaviour in Firefox and using bookmarks in Firefox. Given that OP seems to believe that the only solution is to make sidebar persistent, this answer has changed the approach to offer documentation first and solution last.
Sidebar behaviour in Firefox
Firefox shows the sidebar when the user wants to quickly view Bookmarks or History. The following points describe the sidebar behaviour in Firefox.
- The sidebar is shown on the left (default) when visible.
- If the sidebar is already visible in a window, the sidebar will also appear in a new window (not applicable to a private window).
- If the sidebar is already visible in a window, then the window is closed and opened again, the sidebar will be visible.
The sidebar behaviour has been observed in Firefox 58 and older (notably Firefox 45, 37, 29) and found to behave consistently regardless of Firefox releases.
One exception is Firefox 29: When a tab is opened automatically and redirected to Mozilla page to notify Firefox version is outdated, the sidebar that supposed to remain open will be forcibly closed at startup. However, this is not observed in newer but outdated version i.e. Firefox 50.
Based on above, the sidebar in Firefox will usually remain visible after the sidebar is first opened and not closed by the user. The sidebar non-persistent behaviour is likely predefined and can not be modified by user configuration. No relevant setting in
about:config from my finding.
Using bookmarks in Firefox
Mozilla has explained how to access and organize bookmarks in Firefox, which is found under "Learn the basics: Get Started". Notice that sidebar is not mentioned there. Use the Firefox sidebar to access bookmarks... is explained anyway, but found under "Manage Preferences and Add-Ons".
The intended way to access bookmarks in Firefox is either from the menubar, dedicated buttons on the Toolbar, or Bookmarks Toolbar. Bookmarks are also accessible from sidebar however, the sidebar itself does not seem to be the intended way to persistently access bookmarks.
Sidebar or other problem
As hinted by documentation, using sidebar is actually less intuitive than already visible buttons on the toolbar. For Firefox users, I could identify three use cases:
To access bookmarks from somewhere that is visible but temporary, use either "Library" button, "Bookmarks Menu" button, or the hamburger menu (right-most) button on the Toolbar.
To access bookmarks from somewhere that is visible but persistent, use Bookmarks Toolbar. This is more legible than 1, since each item on Bookmarks Toolbar has favicons and text.
To access bookmarks from somewhere that is accessible by keyboard, use either access key Alt+B to open "Bookmark" menu in menubar or, keyboard shortcut Ctrl+B to open "Bookmarks Panel" in sidebar.
Most end users would likely fall under use case 1 and 2, and some end users who prefer keyboard shorcuts would fall under use case 3. Power users may use the Awesome bar and even Chrome URL to access the bookmarks as preferred, depending on the work flow.
Above all, rather than trying to make the sidebar persistent (may not be feasible after all), try other ways to access bookmarks. The alternative solution is no longer suggested, since the setup was initially assumed to be done by OP and not OP's mother.
TL;DR Back to basics: Read Help Topics for Firefox before using the product. Do not decide from someone else perspective; Explain available choices and let the user decide what is best.