Farrell was trying to get a raise for her final season, but Paramount went behind her back and hired everyone else and ended up firing her. That just rubs me the wrong way. As a result of that, we wasted several episodes developing Ezri Dax during the last season.
Hmm, actually I heard a slightly different version recently from Ron Moore and Ira Behr. Apparently everyone on the cast were given attractive offers for the seventh season except Farrell who - for some reason - was picked on by Paramount and given a poor offer. She tried to negotiate it to the same level as her colleagues but they wouldn't budge, so she quit.