Your question is what is actual meaning of AtharvaVeda Mantras you quoted - translated by Griffith.
Let's look at the verses from another translation of Atharva-Veda by Shri Ram Sharma Acharya.
Atharva-Veda 6 .44 .1 - Roga-Nashana Sukta .
अस्थाद् दयौरस्थात पृथव्यस्थाद् विश्वमिदं जगत् |
अस्थुर्वुक्षा
ऊर्ध्वस्वप्नातिषठात रोगो अयं तव ||
The Dyuloka or swarga is stable likewise this earth which is base of
all the creatures on it is also stable . The trees which sleeps even
they remain standing are stable or not moving. Just like all of these
let this bleeding from wound stop .
Here in this mantra there is a prayer to stop bleeding. If we look at the translation we can see that the actual meaning of firm stands the earth or "Astahat" is not be taken as static but as firm and not frequently changing shape ,form etc. providing a stable , non movable land support and rigid support - which is essential for creatures living on her to live well. i.e. for building homes , agriculture purposes etc.
In the similar way the word "Asthat" is used in
Atharva-Veda 6 .77.1 Which is Pratisthapana Sukta and Griffith translated it as firm stood the earth. Let's have another look.
अस्थाद् दयौरस्थात पृथव्यस्थाद् विश्वमिदं जगत् |
आस्थाने पर्वता
अस्थु स्थान्मश्र्वाम् अतिष्ठिपम् ||
The Swarga , earth and centers of both are stable in their respected
positions and boundaries on this universe. The mountains are also
stable. Likewise let us control our dynamics with our power of soul.
Here in this mantra a advice is given to us by risies to control our senses and to keep them in their respect boundaries for the sake of our own good. Just like Swarga and Earth and their centers are stable i.e. they are not freely moving uncontrollably in this universe also we should not let our dynamic powers (senses) loose. And here stable boundaries of earth and swarga are used as example.
Atharva-Veda 6 .88 .1 – Dhruvo Raja Sukta
ध्रुवा दयौर्ध्रुवा पृथिवी ध्रुवं विश्वमिदम् जगत |
ध्रुवास: पर्वता
इमे ध्रुवो राजा विशामयम् ||
Just like the sky , mountains and entire universe is not deviating ,
let the king of the land be stable.
Here the word Dhruva is used as Synonyms to stable. But the meaning of it is totally different is not unmovable but not deviating , the purpose of the mantra is a prayer for a king to remain fixed in his views and he may not get confused or change his mind frequently . let the mind of the king do not deviate or unstable. firm etc
Now coming to AV 13.1.6 . It’s a Adhyatma –Prakarana Sukta.
रोहितो द्यवापृथिवी जजान तत्र तन्तु परमेष्ठि ततान |
तत्र
शिश्रियेsज एकपादोsद्रुंहद् द्यावा पृथिवी बलेन ||
Swarga and earth are emerged from Surya (sun god) .There Prajapati
developed and broadened the the Atmatattva ( soul element). the Atman
has taken shelter over there. And given the strength to earth and
swarga.
It is mantra from Adhyatma prakarana. i.e. this is spiritual section and eulogy of sun god (surya ) , Rohita (shining sun). This sukta has 60 mantras in which how surya or sun god created the earth and swarga is described i.e. from his various rays , how sun god created various fires and how he established earth as vedika ( alter for Yajna) , due to him rain came etc.
Here surya is considered as soul of this world and in his eulogy its said that he has created whole universe and in similar way he has fashioned earth and swarga. I.e. he created everything on earth like mountains , food , trees etc. from his powers
Conclusion - SO if we look closely at other translators we can find
that all these mantras is having deeper meaning than it appears , and
not to be interpreted as they are. Griffiths translation generally is
word to word and lagging in core message of vedas. Any way the words
like Dhruva and firm are just used to give examples . They does not
necessarily mean to say that earth is static or immovable , but as
shown above each word is used for different purpose , and this does
not mean those are the errors in Atharva veda , or the Mantra-Drashta
Rishi's were believing that earth is static.
