I have a flash (flex) web application and when it first loads the cursos is in the username field blinking. However, you can\'t start typing because Flash does not have focu
The method linked by Zilupe described in the note is for Windows IE only, probably not good enough.
I haven't tried this myself and for the comments on the page it seems it's not reliable, but check this out: Firefox Focus and Actual Links.
This is a very annoying problem that effectively kills any accessibility efforts you might have put in your Flash application. Blind users using non-IE browsers will NOT be able to use your app because they simply cannot click on it to give it the focus.
Last time I investigated the problem I found a comment by some FF developer stating that it was actually Mozilla's fault (sorry, lost the link) but I honestly don't care. Adobe should chase up Mozilla and put some effort to fix this, it's on Adobe's interest.
Check out SWFObject. For security reasons FLASH never is auto focus - otherwise stealing keyboard input would be a snap.
You should use Javascript. Here's an official how-to: http://kb2.adobe.com/cps/155/tn_15586.html