Not sure how much sense that made... so let me explain it differently.
First, I am not sure if Firefox queries the system datetime when a file is downloaded or queries the filesystem or uses it's own routines. I would guess it's one of the first two though.
You download a file today. Firefox gets a date and time through one of probably the first two methods. If day=today, it says "Today" in Firefox's language (English, in this case) instead of showing the date. If day=Today-1, it says yesterday instead of showing the date - again in English, as that is how "Yesterday" is coded into FF-EN (I know of no system call that actually returns "Today" or "Yesterday", so those have to be "hardcoded" into Firefox).
If the file is older than that, it uses the date/time info returned via probably either step one or two above, meaning what you get is the locale based version of the date - in this case, "fr_FR_EURO" - which of course then results in incorrect accents due to the codepage.
I cannot think of a way around this. I would expect any function dependent on a/most system call(s) will return a result in the format determined by the various locale settings (language, date, time, numeric, currency, etc).
Not much help, I know... but it hopefully explains the situation - assuming my assumptions above are correct of course.