I have an AR-300M on which I want to access the NOR firmware (to flash a modified version from Freifunk). After updating uboot, I was able to make the slide switch work (via the uboot setting, but it is reset each time I boot into NAND I think) to boot directly into NOR. However, I cannot log in - neither “goodlife”, nor the NAND password or leaving the password field blank seem to work. There is a “reset” button on the Luci login page, but it seems to have no function. Is there a different default password for NOR?
Alternatively, can I update the NOR firmware in uboot directly or only NAND? There is only one button for flashing, unlike before the uboot update where I could choose whether to flash NAND or NOR.
If you flash the NOR image using uboot you should not have problems. There is a long documented bug with the size of images (bigger than 16Mb) that affect flashing of NAND images, but NOR images get correctly flashed.
The method I adopt to flash both NOR and NAND is by connecting UART to usb, and running the commands available in printenv.
It is less than ideal, but at least it works.
Many thanks. I used the *.bin file and it flashed perfectly in uboot. 
Can I ask you what is this firmware?
In absence of a correct NAND firmware the router defaults on NOR. You will have to flash correctly on the NAND section, if you want the slider to work, and have two working firmwares to choose with slider.
Sure you can. Freifunk is an initiative allowing the secure sharing of your internet connection (in Germany): Freifunk - Wikipedia
They normally use customized OpenWRT images providing a WiFi hotspot and connecting to a VPN to tunnel the user data - so they act like an ISP and the owner of the shared connection will not be held liable in case of abuse.
Regarding the slider issue, that was with the latest stock firmware. My initial uboot version did not allow setting the slider function to choose between NAND & NOR. After updating uboot, it had the option. It worked well as long as I kept booting into (stock) NOR, but every time I’d boot into (stock) NAND it would lose its function and I’d have to go back to uboot and enable it again. No big deal for me as I do not really need NAND.