I would wait at least until Adobe release a beta of flash that supports HW acceleration (10.1), until then use the built-in browser. If firefox for mobile has some advantages, they're with the extensions. Which will slow the browser down some more, but if you need an extension that you need to use, mobile firefox is needed.
Also, when trying to use the firefox as a main browser, the user need to remember that the built-in browser opens all links from emails and IMs, but there is an app that allows you to do change that. I think.