Yes, a 2600 router can do firewall functions in sertail feature sets, those sets need to be bougt and are complete new software images and therefore you should allso watch your memory configuration.
There are 4 versions witch can be bougt with FW features, these are :
IP/FW/IDS
IP/IPX/AT/FW/IDS PLUS BASIC
IP/FW/IDS PLUS IPSEC 56
ENTERPRISE/FW/IDS PLUS IPSEC 56