FF is fine, I use it on W7 with NoScripts
it also happens to be standard in alot of XP SOE's, so calm down. Unessesary crap is bad, but FF is for alot of people worth having.
Also try installing just the printer drivers and not the full software package.
Make sure you eliminate the data and pritner itself as potential issues.
If I understand you correctly, she can currently print from some documents, but not from her PDF reader? I agree that I would first try installing another PDF reader and seeing if the problem persists. Also, I would recommend re-installing the printer (if it was not just detected via plug and play) and this time right-click on the installer and choose 'Run as Administrator,' as it also sounds like there could be a permissions error causing the issue.
When I get a chance, I will attempt all these suggestions. I don't live with my girl and don't see her usually during the week.
It looks like we are going to have to try all these suggestions since the HP and even the Sony people couldn't fix the problem and they literally spent about two days trying to fix it.