In bibliography, problem of displaying French caracters (à = �)

Dear all,
In my citation syle I included the following line for citations with URL :
<text variable="URL" prefix="à la page web : " text-decoration="underline"/>
It works fine, except that the "à" that appears as a � when I cite it inside LibreOffice and as a □ in word.
I found a somehow related thread on this forum:
I tried the solution to change the encoding of Firefox, but it remains the same and I have the same error when I generate it as from the standalone version...
Has someone got an idea on how to fix it?
Thanks in advance,
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