Another candidate for Agnes and Mary's mum (assuming she was not formally married to any of her husbands... )
Francis Matilda FIRTH, born 19 September 1832
*** and baptism in Hobart, VDL, 6 June 1841. Her parents recorded as Joseph Crossley FIRTH and Tabitha FIRTH
https://familysearch.org/pal:/MM9.1.1/XT8C-MR8 https://familysearch.org/pal:/MM9.1.1/XTXX-4D6 Joseph Crossley FIRTH died 24 Nov 1865, aged 75 years, spouse named Tabitha, HOBART, Kingston Congregational KB06/00012 Tas death certs usually have very little family history info on them, but the ref no. is 5972. He had been tried for High Treason in Yorkshire, England. His wife was likely a Tabitha TAYLOR.
*** The online index simply
does not say where this baby girl was born, and so she may well have been born 'overseas' ie Ireland.
Cheers, JM