Can't "merge" the Firefox / Sync accounts via Sync, but the data on each will get "merged" if you connect the "business Account" to a device that is using the personal Account.

But only one Firefox / Sync account can be connected to the Sync server at a time per device or Firefox Profile. So Disconnect from that personal Firefox / Sync account first so that you can connect to the "business Account". And once the "business" is transferred to that device be sure to Disconnect that account on that device.

Keep in mind that any data on that device will synchronize "back" to that "business Account". There is no "one-way" synchronization possible with Sync. The data on the device will get synchronized back to "business Account on the Sync server automatically.

Last time I upgraded Firefox, all my bookmarks vanished, & I do have 2 Firefox accounts as well. Signing into both of them on the same device has not merged my bookmarks, they're still not showing at all. I've always synced my account, but with the last update of Firefox Browser, all my bookmarks are just gone.

Is your Sync data on your other devices which are connected to Sync?