Whenever a page renders differently on different browsers the first
thing to do is to check the source for html validity. View the page
in the browser then view the source (View > Page Source or similar)
copy the complete html text and paste it into the w3c html validator.
It may not be the problem but is always worth doing first.
Apologies, I had missed the fact that FF remembers the setting on a
site by site basis. In which case, with some embarrassment, I
unreservedly withdraw my disbelief.
if you need to reset it you can simply use cmd-0 to reset to default.
They are all together 0 - + right next to each other.
I'm on ff4 and it looks like they implemented = so there is no need to
shift for + and here it's control ( 0, -, +/= ) to control the
browser's text accessibility option.