I have 4 days running r77.20.85 Build 755 and appliance is stable. Just noticed there are a difference between memory reported by webui and top command. WebUI tools report constantly about 700Mb memory usage, while top report 1.5Gb aprox; although I consider that such differences come from previous fimwares.
Leaving aside that, the firmware performs well.
top - 10:27:58 up 3 days, 15:43, 1 user, load average: 1.35, 1.62, 1.78
Tasks: 122 total, 3 running, 118 sleeping, 0 stopped, 1 zombie
Cpu(s): 3.6%us, 3.8%sy, 0.0%ni, 91.7%id, 0.0%wa, 0.0%hi, 0.8%si, 0.0%st
Mem: 2068516k total, 1582132k used, 486384k free, 7496k buffers
Swap: 0k total, 0k used, 0k free, 631624k cached