United and Liverpool face each other on Saturday in the first meeting between the teams since the racism controversy involving Luis Suarez and Patrice Evra which resulted in an eight-match ban for Suarez.
The acrimonious nature of the Suarez affair has stoked one of the most bitter rivalries in English football, even if Ferguson has previously insisted it would have no impact on relations between the two clubs.
In a letter sent to United fans who have tickets for Saturday's encounter at Anfield, Ferguson called on United supporters to be on their best behaviour, calling for "positive, witty and loud" support.
"FA Cup ties are tense affairs at the best of times," Ferguson wrote in the letter, without referencing the Suarez saga.
"Add in the fact that Manchester United against Liverpool is the biggest game around and it becomes even more so.
"Your support is vital to the team and down the years that has been especially true at Anfield.
"But please put the emphasis on getting us into the next round and giving the sort of support you are famous for - positive, witty and loud."