I've been using it for a month or two on BobZ's CM9. It looks a lot better than any other browser because it uses a much higher-quality downscaling algorithm. Not sure exactly what they're doing, but it appears to be more advanced than simple bilinear or bicubic scaling. It's very good about preserving gridlines at any scale, without even blurring them. This has long been a sticking point for me with all Android browsers, so I was really impressed. I'm honestly not sure how they made it look so good. Better than iOS, even.
The downside is that this comes at the cost of performance, which is only natural. I found rendering, zooming and scrolling to be significantly slower in Firefox than other browsers. The performance eventually drove me back to the stock browser.
Also, it zooms WAY too far when you double-tap. This really hurts usability.
Edit: I just checked the Play Store, and....it says it's incompatible. Very strange. I used it for quite a while before, even getting updates from Google Play. No idea what happened there. I still have the last version I installed on my phone, so quite sure I was not hallucinating.