I encountered this problem while accessing OBIEE analysis using FireFox 12.
Browsing internet provided two solutions:
1) Apply official patch
Since this is a recognized problem the best way to solve this problem is to apply official patch (available for Linux).
Log on to:
Transfer it to the OBIEE server then unzip it
Apply patch using OPatch:
[oracle@orcl ~]# /home/oracle/obiee/oracle_common/Opatch/opatch apply /home/oracle/13564003/
/home/oracle/obiee/oracle_common/Opatch/opatch is the path to the folder of OPatch application
/home/oracle/13564003/ is the path to the folder created when downloaded patch was previously unzipped.
After patch installation restart OBIEE server.
2) Fake User HTTP Agent
Although the solution above is preferrable if it's not possible install the previous patch there's another way to solve this problem.
BI Presentation Services seems not to support
User-Agent = Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0) Gecko/20100101 Firefox/10.0
so changing it to a lower version using some Firefox addons like "User Agent Switcher" did the trick.
Open Firefox, type:
Click right mouse button on the page and select "New->String".
And use this value:
Mozilla/5.0 (Windows; Windows NT 6.1; rv:10.0) Gecko/20100101 Firefox/9.0
Refresh OBIEE webpage and everything should work!!