As usual it a a matter of what you trust, and what threat you want to mitigate. You should never store private passwords on a shared computer for example.
If you trust your browser to correctly read URLs (anyway if you do not, you really should not use it!) you assume that it will only autofill a password on the right site, so the only problem is the storage of password on disk.
It is as secure as (the weaker of both):
- the encryption implementation used by the browser
- the master password you use
provided you consistently close you browser and/or lock the screen when you leave you computer. Anyway the latter one is a good practice unless the room containing the machine is physically protected.
My opinion is that I can trust enough a raw Firefox to store most of my passwords in it with a global master password. I would not store a highly sensitive password, and the for the browser part, the more add-ons the less trust...
TL/DR: My opinion is that it is fine for general use passwords, not for the ones that give access to sensitive national security informations.