SJCL was used for SRP because that's what Apple's extension uses too.
It's required for AES encryption/decryption, SHA256 hashing, big numbers, bit array manipulation, binary/hex/base64/utf8 transcoding, and a bunch of known group constants.
Those are all very standard, so that should be feasible with the browser's built-in libraries or a very lightweight library.
Originally posted by @aurelien-garnier in https://github.com/aurelien-garnier/icloud-passwords-firefox/issues/16#issuecomment-1802204552