What is this ?

bookmark in the toolbar starts to refuse to appear

No idea what your talking about.

Do you you mean the Menu Bar ?

So know what Bookmark Toolbar is but you have lost me on the other.

Please try : SAFE MODE

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

In Firefox Safe mode these changes are effective:

• all extensions are disabled (about:addons)
• default theme is used (no persona)
• userChrome.css and userContent.css are ignored (chrome folder)
• default toolbar layout is used (file: localstore-safe.rdf)
• Javascript JIT compilers are disabled (prefs: javascript.options.*jit)
• hardware acceleration is disabled (Options > Performance > Uncheck to view)
• plugins are not affected
• preferences are not affected

TEST''''is issue still there ?

The Bookmarks Toolbar is not displayed by default, so just in case that's the issue, you can use one of these methods to show the toolbar list and select the desired bars there:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key to activate the menu bar temporarily.
• in Customize mode > Toolbars button (see: Customize Firefox controls, buttons and toolbars)

To turn on the Bookmarks Toolbar, Menu Bar, or other bars, click it on the list.

Assuming the bar is turned on:

• Bookmarks Toolbar is there but it's blank
• Bookmarks Toolbar is not visible at all below the address bar

For the first problem, the "Bookmarks Toolbar Items" control may have been removed from the bar (either moved to a different bar or not on any bar). You hopefully can fix that using the Customize feature. See: Customize Firefox controls, buttons and toolbars.

For the second problem, some users of Firefox 61 reported the bar would be present at Firefox startup, but then would disappear for the rest of their session. I don't recall any quick fix, but this thread has my suggested things to try as well as a link to a longer thread: https://support.mozilla.org/questions/1228692

Thanks for the response guys. Sorry if I didn't make my problem clear.

I've activated "View bookmarks toolbar". Before the update, if I do this, the folders where I put all my favourite sites will show in the toolbar under the address bar so I can quick access. But now even it is activated, nothing is shown.

Joefucious said

But now even it is activated, nothing is shown.

Hopefully the second part of my earlier reply is helpful for next steps.

Problem is solved. Thanks a million.