If I remember correctly, she was pissed because they used some clips of her in a different episode, without her permission. That's why she declined to give them permission for the finale.
Not quite. It was Terry Farrell's
agents who complained about unauthorised use of Jadzia clips in a previous Season Seven episode - as was their right - and
because that issue was still pending in arbitration, she
couldn't legally be approached for permission for more clips in the finale. However, she
was invited to the DS9 Season Seven wrap party - and attended cheerfully - and was on good terms with the writers.
It was the salaries department, who treated her badly, that led to her departure at the end of Season Six. She was the cast's representative at negotiation time, pressuring the studio for big raises for all. The studio went behind her back and made separate deals with each actor (bigger trailers, etc), then offered Farrell an amount that, with inflation, was essentially less than she got in Season Six.