Firefox has a way to set a primary password, however IIRC there are some major issues with it. However, it is worth considering what you are trying to protect

Firefox has a way to set a primary password, however IIRC there are some major issues with it. However, it is worth considering what you are trying to protect against and for most single user systems I'm not convinced it is worth using something other than the built in browser password manager (unless you have a number of other applications that require passwords and don't have an easy way to store them). Encrypting paritions with sensitive data is a better way to protect data when the system is off. Unless you clear cookies all the time there is quite a bit that can be done just with cookies, although protecting passwords should at least prevent loosing access to accounts. In some cases a password manager can help with the possibilty of a computer being stolen while on. Uploading unencrypted files right away is easier if someone gains remote access but, while depending some on the specific OS and password manager, it is usually not too difficult to start reading passwords as they are used and intercepting the primary password of the password manager the next time it is used might not be all that hard either.