Ah, yes, I was being unclear here: the "without prior notice" part was not meant in the legal sense, i.e. firing an employee immediately due to egregious misconduct, but the way it was delivered, on stage, right after the final performance of the tour. That is a point in time picked precisely for one thing, to hurt as much as possible. It was payback - for slights perceived or real, well, that is something else, but you'll get no argument from me that Tarja most likely had it coming, but I did and do think that the public spectacle the firing became was unbecoming and unprofessional of the group.
How they dealt with Anette later just compounded that impression.
No, that is probably right, there has to be a reason why the rest of the group basically made it about "remove Tarja or we'll leave", but regardless, that is not a reason to do this in such an unprofessional manner, and from what I have read over the years, she wanted to pursue other interests and would have left anyway. It logically followed with the rumor mill endlessly churning on about how Tuomas fired her over unrequited love. Not that I doubt that he was actually in love with Tarja, as Ever Dream is as good a love letter to Tarja as any, but the firing was certainly not over that.
Anyway, that Tarja did not really want to be a part of the group any more was fairly evident for me in two things, first, I watched the End of Innocence documentary, which contained a lot of rather subtle clues about what she was thinking at the time, and second her attitude at the concert I was back in 2004, where we actually managed to stumble upon the group after the concert on their way to the tour bus. Sure, it was a long day as there were ridiculous promotional events all day long prior to the concert (at which she appeared to not really want to be there, basically half-assing her performance for much of the show), but the guys were not bothered by being asked for another round of autographs, and even agreed to taking some pictures. Tarja? Rolled her eyes, scoffed, and ran off.
Okay, I get it, that is exactly the reaction I would have had if that happened to me after a hard day's work, but still, short of directly stating it, it's hard to telegraph "I do not want to be here" any clearer, and if that attitude translated into her behaviour on the tour, which is undoubtedly has, then it is no wonder that she was removed from the group.