I basically went through the same thinking process myself. I'm a moderate idealist and would like Firefox to succeed because it's important for the web that there's no monopoly. However, my realist soul tells me that Firefox is losing badly and it's hard to tell where it's going to end. It's a pity that Microsoft didn't choose Gecko – I wonder if they even seriously considered it.
With Microsoft joining Chromium, we're slowly entering a duopoly of WebKit and Chromium. It's still a much better situation than the dark IE6 times. However, it's hard to tell whether WebKit (AFAIU, supported mainly by Apple and Samsung) will keep up with Chromium (Google and Microsoft) in a long term. I'm afraid that it will look like a Cold War's arms race – one of the parties will invest so much that the other will just lose economically (not being able to afford that many developers). Such economical wars make developers happy, because the web progresses very fast, but I think it actually leads to a monopoly.
Anyway, I'm still keeping my fingers crossed for Gecko. They do a great job there and perhaps will be able to partner with a big enough company to keep its pace and market share.