In the UK there are various types of DSL line and router than can be used. Any device that supports PPPoE can work with the FireBrick, but some options are only available with some devices, as listed below :-
For other types of lines in the UK, or those in other countries, you need to know what they can do on the wire (PPPoA or PPPoE) and have a suitable modem/router to talk that protocol and convert to PPPoE on the LAN link to the FB2900. It seems most DSL routers will bridge PPPoE on the wire to PPPoE on the LAN, but few will act as a PPPoE access concentrator. The Vigor V-130 is one of the few that handle PPPoA on the wire and PPPoE link to the FB2900.
A significant benefit of the Vigor V-130 is that it works with no configuration on BT 20CN and 21CN lines as well as other carrier PPPoA lines and TalkTalk lines - you just plug it into the line and the FB2900 and it just works. There are also modems that work in bridged mode, and support baby jumbo frames allowing PPPoE through to the carrier BRAS with full size MTU.
For fibre to the cabinet (FTTC) and fibre to the premises (FTTP) service you connect the FB2900 directly to the service (BT supplied modem) with no extra equipment.