The fix was quite simple, I had to ensure that in the Windows date time settings (double click the clock in lower right corner), I had checked the checkbox for 'automatically adjust for daylight saving time'. Once this was checked, a firefox restart cured all the weird behavior.
We have a university wide GPO for the domain that forces the clocks on the computers to be auto-synced to the AD server. With this GPO, in 4.5.0, calendar entries were getting the off by one hour error in March. So, we had unchecked the 'auto adjust for daylight savings' which fixed this in 4.5.0, but this *MUST* be checked for 4.5.1 to work properly (if you have a GPO that forces computer time settings).
Of course Mac's worked fine, and all XP machines that were *NOT* in the domain (and did not get the GPO) were fine :-)
Also worth noting, please read the 4.5.1 release notes as there is a bunch of info in there about the timezone changes.