While waiting from a fix from either 'formerly Macromedia' or the Firefox team, here's what I use to work around the float() deficiency on European systems with Firefox 1.5:

Instead of using float() to convert a string to a float, I am now using myFloat(). myFloat() first checks if the current system is affected (seeing if a float is actually a float), and if so changes periods to commas.

on myfloat aValue
 case aValue.ilk of
   #float, #integer:
     return float(aValue)
   #string:
     if not floatP(float("1.0")) then
       tLength = aValue.length
       tOffset=offset(".", aValue)
       repeat while tOffset>0
         aValue = chars(aValue,0,tOffset-1)&","&chars(aValue,tOffset 1,tLength)
         tOffset=offset(".", aValue)
       end repeat
     end if
     return float(aValue)
   otherwise:
     return VOID
 end case
end myfloat

Unfortunately, I have found out that the float() and value() methods are not the only things that break in Firefox 1.5; If you use behaviors with a propertyDescriptionList that includes floats (ie: behaviors with fancy dialogs to set float properties), the stored values will be wrong on affected Firefox 1.5 browsers. No workaround yet, other than not using floats in propertyDescriptionLists.