What add-ons are you using?

Make sure the add-on is active in the Add-ons Manager.

Some have icons that need to be placed on the toolbar.

Is this the exact extension that you have? https://addons.mozilla.org/en-US/firefox/addon/download-flash-and-video/

Viewing the screenshots on the download page, there is supposed to be a toolbar button for that extension at the right-end of the Navigation Toolbar, but those screenshots appear to be for an older version of Firefox, or those screenshots were made with the CTR extension installed / active. Maybe the Toolbar button needs to be moved to a Toolbar?

https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars Open Customize and look there for the Toolbar button for that extension.

I JUST REFRESHED FIREFOX AND REINSTALLED FLASH DOWN LOADER ADDON AND THEN IT SHOWED UP.

So glad that I took the time and effort to post a well thought out solution ....