As NetWeaver 7.0 and 7.3 do not support Firefox 4, there is also no support for Firefox 5. The findings and workarounds of One more browser: Firefox 4 are still valid.
When you run an EFP or your users can install browsers your life will get a little bit more complicated. You can block access to your portal / Web Dynpro, but this approach will leave a bad impression. Once again, company installations of Firefox are safe.
The number of users still running FF 3.5 is impressive: > 12 millions. But Mozilla is planning to get rid of FF 3.5 the hard way: forced upgrade. FF 3.5 users will be upgraded to FF >= 3.6. The process is outlined in the EOL document for FF 3.5 and in the FF forum.
The FF 3.5 release is marked as unsupported as of 3.5.19 in Wikipedia but I didn’t find the same statement from Mozilla. Currently, FF 3.5 is still appearing in the PAM.