firm: not liable to give way, so it holds good and can be counted on
This word describes something that stays put when weight is put on it. Greek writers used it for an anchor that will not drag, a footing that will not slip, and a signed agreement that will hold up if anyone challenges it. The picture behind every use is the same: lean on this, and it will not give way.
In the New Testament that one picture turns up in three settings:
- something you can count on not to fail, such as an anchor, a hope, or a message that has been tested
- a promise or a document that is legally in force, so it can be held to and enforced
- a grip that is kept up to the very end without slipping
The legal side is the one most easily missed. Everyday business papers of the first century used this word, and the matching verb "to confirm" (βεβαιόω), for guaranteeing a sale or making a seller's title good. Hebrews works that language hard: a will only takes effect once the person who wrote it has died (Hebrews 9:17), and the law given through angels "proved binding", so every breach of it drew a penalty (Hebrews 2:2). Paul builds on the same footing in Romans 4:16, where the promise rests on trust rather than on law precisely so that it will hold good for everyone in Abraham's family.
Once, in Hebrews 6:19, it is paired with another adjective meaning "safe" (ἀσφαλής). Together the two describe the hope Christians hold as an anchor: safe, because nothing can get at it, and firm, because it will not shift. The words are close in sense, and the pairing is there for weight rather than for contrast.