Some extensions will interfere with the icons of other extensions...

You might be able to find them and restore them to their preferred locations using the Customize feature. This article describes the steps: Customize Firefox controls, buttons and toolbars.

If you do not usually display the Add-on bar, that's also worth checking. It will display in customize mode, but may then be hidden again once you finish customizing. You can turn toolbars on/off in regular mode using either:

• right-click a blank area of the tab bar (or the "+" button on the tab bar)
• View menu > Toolbars

Any luck?

Thanks, this worked perfectly.

I've always displayed the add-on bar too, but it's at the bottom of the page, and it has half a dozen other icons on it.

So I just experimented and found I could put the lost icons either there or at the top. Cool!

Well, I'm still happy with it, but I've found out that the icons I put into the top area disappear when I close/reopen Firefox. Those I put on the Add-on bar stay there.

If you wouldn't mind, please report this problem to the author of the poorly behaved extension. Hopefully it will be fixed for the benefit of all its users.

Done.

Here is something you can try, create your own toolbar. Once you have created the toolbar, you can put whatever you want on it, and leave the bad icon whereever it wants.