Whosoever is eligible to act as judge is eligible to act as witness, but one may be eligible to act as witness and not as judge.
The discussion that is engendered by this statement relates to the role of women in the Jewish courtroom. It appears that women can act as judges, inasmuch Devorah the prophetess served in that capacity, as is clearly stated in Sefer Shoftim (4:4), “Now Devorah, a prophetess, the wife of Lappidoth, she judged Israel at that time.” At the same time, women are not ordinarily accepted as witnesses (as we learned on yesterday’s daf, there are some circumstances when we will accept their statement, more for purposes of clarifying information than as actual testimony).
In their first answer Tosafot explain simply that our Mishna is only referring to men. Any man who can act as a judge can serve as a witness, while there are some men who can act as witnesses but cannot serve as judges.
In their second answer Tosafot argue that Devorah was a unique case, since she acted as a judge with Heavenly approval.
In their third answer Tosafot suggest that Devorah did not truly act as a judge, rather she taught the laws of judgment to the Jewish people.
The Ran offers a different approach. According to the Ran, Devorah actually was a judge, but she served in this capacity only because she was accepted as a judge by the Jewish people, much as litigants can agree to allow a relative to serve as a judge in their case. Such a position does not give Devorah, or, indeed any other woman, the ability to act as a witness.
In his commentary, the Arukh LaNer argues that there are hints to all of these different explanations in the story of Devorah that appears in Shoftim.