If you want to log in to multiple AWS accounts, the best solution I have found is a thin shell script wrapper around `aws-vault login` [0] that opens Chrome with a dedicated profile. aws-vault stores an API key in your local keychain, fetches a temporary session key on demand, and logs you in automatically.

I've put the shell script I use here:

https://gitlab.com/snippets/1979112

You should be able to adapt it to Firefox pretty easily.

[0] https://github.com/99designs/aws-vault