The Brazilian forward has sat out the last two fixtures with a hamstring injury that also forced him to withdraw from his country's 23-man squad over the international break.
But the 24-year-old has joined the rest of the Reds' travelling paty and was put through his paces with his teammates at the Westfalenstadion on Wednesday evening, coming through a full session unscathed.
Firmino was forced off in the Reds' second-leg draw with Manchester United in the previous round of the competition, complaining of discomfort in his hamstring, missing the defeat at Southampton and the home draw with Tottenham Hotspur.
But his absence for the first-leg against a much-fancied Dortmund side who are the favourites to go on and win the tournament would be a huge blow for Jürgen Klopp.
However, despite a three-week spell on the sidelines, Firmino took part in the entire hour-long open session in front of the media having done their tactical drills back at Melwood in the previous days.
Lucas left in Liverpool with Benteke also out
Elsewhere, only Lucas Leiva is absent from the squad - staying on Merseyside to continue recovering from a thigh injury which has kept him out since the Reds lost the Capital One Cup final to Manchester City in February.
Jordan Rossiter is out with a long-term hamstring issue, whilst striker Christian Benteke's knee ligament injury could keep him sidelined for the rest of the season - damaging his Euro 2016 hopes for Belgium.
Dortmund have few injury problems for the first-leg, with Mats Hummels given the green light to feature although Neven Subotic is unavailable and is unlikely to feature again this term.
Ilkay Gündogan is back in training but isn't expected to feature with this game expected to come too soon for the German, with former Liverpool midfielder Nuri Sahin in contention to play.