How was the behavior of Agni before he was cursed by Bhrigu to be "Sarva bhakshakha" (consumer of all things)?
After the curse of Agni by Bhrigu, Lord Brahma told Agni this:
The flame that is in your viler parts shall alone eat of all things alike. The body of yours which eats flesh (being in the stomach of all carnivorous animals) shall also eat all things promiscuously.
Before the curse, Agni was apparently not in the stomach of animals as the Vaishvanara Agni, which causes digestion.
There was no mention of Agni getting cursed by Bhrigu in Rig Veda. Agni used to consume everything even before that.
Rig Veda I.98.2 eulogises Agni as Vaiśvānara
Present in heaven, in earth, all-present Agni,—all plants that grow on ground hath he pervaded. May Agni, may Vaiśvānara with vigour, present, preserve us day and night from foemen.
Agni was eulogised as the God in the very first hymn of Rig Veda.
May Agni, sapient-minded Priest, truthful, most gloriously great, The God, come hither with the Gods.
Then how can he be cursed by a sage?
In Puranas, Bhrigu was stated to be the son-in-law of Daksha. Even before the incident of Bhrigu cursing Agni, people used to live and die. And, Agni was the medium for their day-to-day affairs.
This must be an interpolated story.