On blogger.com, there is a section of help pages. They have one page (Blogger Help > Blogger Basics > Blogger Browser Matrix) that show what browsers are supported by blogger.com. The page also has some usage data for blogger.com users (although there is no date, so this information could be out of date).
A couple of interesting notes:
1) Only 71% of their users use Internet Explorer on Windows, which blogger.com notes is a lower percentage than the general Internet population.
2) There is also a note that Firefox is the browser everyone uses internally at blogger. com.