The “Equalizer For YouTube” (EFY) icon is no longer there on my MacBook Air

I have “Equalizer For YouTube” (EFY) installed on my MacBook

Air and am accessing YouTube using the Firefox browser. The little EFY icon has

always been right below the song title on the YouTube page. Problem: when I was

listening to music this evening (on YouTube) and wanted to make an adjustment

to the sound, using the equalizer, the equalizer icon was no longer there. I’ve

searched high and low for a solution, but have no idea where the icon went, or

how to get it back. I’ve checked my extensions page and it does show EFY as

being there—and enabled—but nothing shows up on my YouTube page. Help...

[Re-Titled by Moderator]