I posted earlier on a problem with  Firefox 3.5/6, caching and etags headers and Blackboard 7.3. 

The issue appears the second time they click on the xid-9999 redirect which is shown when a file resource appears in a content list.  The message is 

400 BADLY FORMATTED REQUEST, Tomcat / Apache error message

Possble solution - have apache intercept and rewrite the headers for urls in this format.  Check that mod_headers is enabled in apache and add the following to the apache config file.

<LocationMatch "/bbcswebdav/xid-*">
    Header unset If-Modified-Since
    Header unset If-None-Match
</LocationMatch>