The Brazil international scored the decisive goal as the Reds ran out 2-1 winners at Wembley, but was forced off after Vertonghen, while attempting to shield the ball, poked him in the eye.
He recovered quickly, however, and was able to feature in his side's thrilling 3-2 Champions League victory over PSG later that week.
Nevertheless, Firmino says he was terrified he might go blind.
"I was scared of going blind in one eye and not being able to see again," he told Sky Sports.
"Thank God nothing like that happened and it gradually improved day after day.
"Of course, I've been taking my medication and looking after it. Your eyes are very important as a professional footballer. Thank God everything is OK now.
"I was frightened, but the pain gradually passed after the doctor gave me a painkiller and it got better.
"When the incident happened, everything was blurry and I couldn't see out of my left eye.
"It's much better. The pain has gone and I can't feel anything anymore. It's still a bit red, but I hope that clears up over time."
Firmino will hope to feature against Manchester City this weekend as Jurgen Klopp's side look to leapfrog Pep Guardiola's men at the top of the table.