When Franklin first said that, the "those" he was talking about was the Pennsylvania Assembly. They were trying to impose taxes to raise money to defend the frontier.
The Governor kept vetoing these attempts, because the taxes would affect the Penn family.
Franklin wrote a letter to the Governor objecting to this, and included that quote.
The "essential liberty" he was talking about was the liberty of the Assembly to exercise its power to impose taxes even if they would affect the Penn family. The "purchase" he was talked about was literally the buying of weapons to arm those defending the frontier.
Later, he used that quote again but with more the sense that most people use it for now. Someone on Reddit once gave me a reference to where he did that, but they were one of those people who deletes their posts after a while and I didn't save the link in time.
I probably should have mentioned that the Penn family offered to give a lump sum toward frontier defense in exchange for the Assembly acknowledging that it did not have the power to tax their lands.
This change (for better or worse) was motivated by multiple cases where wrongly accused individuals narrowly avoided losing their liberty due to the last minute discovery of exonerating gossip and notes on the accuser's phone. There isn't a strict black and white spectrum on this issue when it comes to liberty, because it concerns cases where one individual is seeking imprisonment of another individual.
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety."