Simple words like parler, aller, monter...how were they pronounced before the final R became silent? And when did the ending become silent, anyway?
They were pronounced with a "soft r", [parleɾ], [aleɾ], [mɔ̃nteɾ] vs the current [parle], [ale], [mɔ̃te].
The ending [ɾ] fade out between the 16th and the 17th century.
All infinitives in -r were affected, so verbs like partir were pronounced [parti]. The final r pronunciation was restored for all verbs but the first group ones starting from 1750, but that r was a strong one [r].