Proxy at Home vs work

This may be a FF specific question, but...

I just started a new job. My U3 PFF by John works great at home, laptop and at my old job, but...

When I plugged into my new computer at the new job I found that I could open PFF, but I could not connect to certain sites, like yahoo and google, the only one that will work so far is google mail.

PTB works just fine. Filezilla works.

If I use proxy settings then can I switch them easily every time and is that what I need?

Any suggestions for getting PFF to work?

BTW I did search and could not find a problem like mine, plus the multiple location thing.

UPDATE:

This is the message that it gives.

I know it can't be specific sites though because I can access them with IE.

403 Forbidden - The ISA Server denies the specified Uniform Resource Locator (URL). (12202)
Internet Security and Acceleration Server