This happened to me today. Using FF 3.0, I downloaded a .tgz file *but* firefox gunzip'd it after the download but did not rename the file.
$ tar tzf <filename>.tgz gave errors $ tar tf <filename>.tgz listed the file in the tar archive!
Strange thing is I downloaded other files with .bz2 compression but FF left them as is i.e. in compressed format.
The OP must have gone through this strange experience as well.