I had a similar problem before but, I dealt with it till the official release. I'm on Ubuntu 8.04 which came with FF3 Beta something or something. I could type in the text boxes but, I couldn't go back and alter text without backspacing back to the error and I could not type after a
when replying to it.
I also couldn't paste text. After FF updated itself to the official 3 release, all was returned to normal.