I´m a bit confused. Lesson 2 (page 23) of The Valfells and Cathey
_Old Icelandic: An Introductory Course_ book states in the vocabulary
that "þeira" is the genitive plural form of the pronoun (I
assume "personal pronoun") and can be translated as "their". The
chapter says nothing about the form "þeirra".
However, in the text section at the bottom of the page both the
forms "þeira" and "þeirra" occur:
1. Vápn þeira eru sverð, spjót, ok skjöldr.
2. Matr þeira er ostr ok brauð, fuglar ok fiskar.
3. Mjöðr ok aðrar veigar eru drykkir þeirra.
4. Konungr þakkar þeim kvæði þeirra ok allir drekka ok
eru glaðir.
In 1, þeira refers to víkingar in the previous sentence.
In 2, þeira refers to jarlar, hersar, and víkingar in the prev.
sent.
In 3, þeirra refers also to jarlar, hersar, and víkingar.
In 4, þeirra refers to mörg skáld in the previous sentence.
I have used the symbol ö to stand for the o with an ogonok (tail).