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].
Sources:
http://monsu.desiderio.free.fr/curiosites/r-phono.html
http://ycmorin.net/wp-content/uploads/2012/11/2012ms-R_final_XVIe_siecle.pdf
http://ycmorin.net/wp-content/uploads/2012/11/2006-Histoire_phonologie_graphie_du_fran%C3%A7ais.pdf
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