hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Have you checked your customization menu to see whether is that icon there or not?

Try to click Restore Defaults in the Customize palette window.

• "3-bar" Firefox menu button > Customize
• View > Toolbars > Customize

• https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars

thanks :) It was really helpful... and now i have a prettier browser too :)