Hi, I've got a couple of users that use firefox in place of our SOE version of IE, while I haven't seen anything specific in firefox that causes issues, the version of Java JRE can cause strange behaviour in the webclient. Note, we are using same version of ARIS.
Check if you are using JRE version 7 - I found that to cause inconsistent issues from one user to the next, some had missing interface / objects, others could not use the Evaluation functionality to run reports, and others again had scroll bar issues and couldn't navigate correctly.
Once we'd disabled the later version of JRE, all these issues disappeared so maybe start with that - return your client machines to a supported version of JRE and try again.
Hope this helps.