Not just Safari and Firefox, on iOS all browsers are forced to use Safari's web view, because Apple wants to force developers to write apps so they can make 30% revenue from the app should any money exchange hands. They can't force developers to write apps if web browsers on iOS are allowed to access bluetooth and other modern browser APIs.

It’s important to note that it’s not a matter of effort for Firefox. They’ve decided that the it’s not something they want to implement[1]. The reasoning is that they think it allows low enough level access to potentially mess with devices who weren’t made to be resilient to malicious input, and didn’t like that the proposed method of allowing web Bluetooth is based on a default allow policy with a blocklist, which means as new Bluetooth device vulnerabilities are discovered, this blocklist has to be maintained.

[1]: https://mozilla.github.io/standards-positions/#web-bluetooth