Run the downloaded file through Stuffit Expander first.
Check for the app in
Macintosh HD:Applications:Internet Utilities:Aladdin Folder:Stuffit Expander:
Launch Stuffit Expander and choose Expand from the File menu. Choose the downloaded Classilla file (it should have a .sit extension). It should then decompress and appear as a folder in the same location as the download (usually the Desktop).
If that doesn't work and IE has mangled the download then try downloading on another computer and copying the .sit file across.
A lighter alternative is to download iCab 3.0.5 for Mac OS 8/9 at
http://www.icab.de/dl.php - It's not as compatible as Classilla, but better than IE and may get you to where you need to go.