Did you check for an overflow (>>) button at the right end of the Navigation Toolbar that might store those lost icons?

Try to rename/remove the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Which add-ons are you talking about? Some of them don't work with the newest Firefox.

cor-el said

Did you check for an overflow (>>) button at the right end of the Navigation Toolbar that might store those lost icons? Try to rename/remove the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I have no overflow button anymore. Only on the bookmarks toolbar. I renamed the xulstore file to no avail then removed it and still no success. I restarted each time.

Try to reset browser.uiCustomization.state on the about:config page.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

I found the file and reset to default. No changes after resart.

Answer to Fred McD's question above? Have you looked for the missing icons in the Customize Box? [Click on "Customize" at the bottom of the Three Bar menu].

not there

FredMcD said

Which add-ons are you talking about?
Some of them don't work with the newest Firefox.