adding iceweasel to the list of firefox binaries to find on linux.
removing the really outdated ones we don't support, nor should we support the mangled naming.
(I can't believe if someone had 'firefox2' in their path it would choose it FIRST, even before 'firefox')
Raise an exception when the binary is not found, so the user gets a nicer message.

Fixes Issue 4940.
Fixes Issue 5075.