SGI has posted the new Mozilla 1.4b binaries on their Developer Central Open Source page. Apparently it's been available since the 12th though the Mozilla project news page hasn't been updated - I just happened to check the FTP server and stumbled across it.
They also gzipped the tardist again (same as with 1.4a) so Software Manager won't know what to do with it unless you decompress it beforehand (i.e.: gtar xvfz fw_mozilla.tardist).
Download:
ftp://oss.sgi.com/projects/mozilla/download/irix/1.4b/fw_mozilla.tardist
Seems to have lost the spell checker in c omposer that 1.3 had!
It will probably be included in whichever version is current when August Freeware is released; the builds on OSS tend to be very basic.