The result leaves Jon Whitney's men 11th in League One.
Phil Brown’s Southend made the brighter start and threatened first on five minutes, Anthony Wordworth flashing a low left-footed drive wide from 25 yards.
The Saddlers suffered an injury blow on the quarter-hour mark as Erhun Oztumer, unable to shake off an earlier knock to his ankle, was forced off and replaced by Moussa.
The home side wasted a good chance to go ahead on 17 minutes when the unmarked Ranger headed badly over from John White’s precise right-wing cross.
And Southend were made to pay for that miss as Walsall took the lead barely 60 seconds later.
Joe Edwards reached the byline and accurately cut the ball back for ex-Shrimper Moussa to calmly fire first-time beyond home keeper Ted Smith.
The same two players were again involved as the visitors – chasing a first away win on 2017 – made it 2-0 seven minutes later.
After Edwards’ low centre wasn’t dealt with by Ryan Inniss, Moussa coolly placed a right-footed effort into the bottom corner for his second goal of the afternoon and fourth in Walsall colours.
Southend could have reduced the arrears nine minutes before half-time, but Neil Etheridge produced a spectacular finger-tip save to flick Wordsworth’s sweetly-struck volley over the bar.
Half-time: Southend 0-2 Walsall
The Essex club did manage to halve the deficit just past the hour, Ranger powerfully heading home Will Atkinson’s cross after the Saddlers were caught cold by Ben Coker’s quick free-kick.
Southend came within inches of an equaliser moments later as former Saddler Jason Demetriou’s dipping 25-yarder whistled a fraction wide.
The Shrimpers continued to dominate with Edwards clearing a Theo Robinson attempt off the line.
Substitute Robinson again went close, the forward rolling his man and curling a right-footed effort towards goal but a flying Etheridge acrobatically tipped the ball behind.
And the pressure told on 76 minutes as Wordsworth’s free-kick dropped invitingly for Ferdinand to volley into the top corner.
The comeback was complete seven minutes from time, Cox curling a superb free-kick into the top corner from the edge of the box.
The hosts had chances to extend their lead, with Ranger stinging the palms of Etheridge and Robinson bending a long-range left-footer narrowly over.
Cox fired straight at Etheridge from Robinson’s pull-back in added time but the Blues held on for all three points.
Full-time: Southend 3-2 Walsall
Team: Etheridge, O’Connor, Preston, Laird, McCarthy, Chambers, Osbourne, Dobson, Edwards, Oztumer (Moussa 14’ – Morris 81’), Makris (Bakayoko 88’).
Subs not used: MacGillivray, Jackson, Randall, Roberts.
Attendance: 6,756 (228 Walsall supporters).