I have filed a bug report about this issue. I will add this post to the list.

Please do not comment in bug reports. Bug 1155413 - Many icons missing from Add-on Customize Manager https://bugzilla.mozilla.org/show_bug.cgi?id=1155413

Do you see any error messages in the Browser Console (Firefox/Tools > Web Developer) after stareting Firefox related to this extension or other extensions?

• https://developer.mozilla.org/Tools/Browser_Console

If missing items are in the Customize palette then drag them back from the Customize window on the toolbar If you do not see an item on a toolbar and in the Customize palette then click the Restore Defaults button to restore the default toolbar setup.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.json, extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search bar to check if there is a compatibility update available.

If this hasn't helped then also delete possible addons.json and addons.sqlite files.

@FredMcD: Thanks a lot @cor-el: No, there aren't any error messages Yes, I tried the restore button, but with no other result, the icon is still missing in the toolbar and in the customize palette I will try to delete the extensions.* and compatibillity.ini and come back for feedback

Feedback: Deleting the extensions.* and compatibility.ini did not help and deleting the addons.json did not help either. So I think I have to wait until the bug is fixed :(