Launch icons lost from taskbar when rebooting

I have been using Zorin9 for some years. I recently changed my computer and reinstalled Zorin9, setting it up using Wolfmans excellent 'Post Instalation Guide' much as before. However, I have a small problem. I've attached some program icons to the top taskbar (such as Firefox & Thunderbird). They work fine, but everytime I have to re-boot the computer, the icons disappear from the taskbar. If I put them back (by dragging & dropping desktop icons to the taskbar), they work fine again - until I have to re-boot. Is there a way of preventing this happening?

Hi, instead of dragging to the panel, open the applications you want to 'stick' then right-click the icon in the panel and select 'pin application' or something similar - can't remember exactly the terms but it should be there in the context menu.

Thank you for the suggestion. Unfortunately right clicking the icon doesn't give a 'pin application' type option. I've had a look on the Ubuntu site but found nothing of use. It must be possible as I didn't have this problem previously. Fortunately I don't reboot that often, so it's not that important. Thank you anyway.