Is Firefox’s password manager generally safe for non-critical logins?

I have separate passwords for each forum I participate in, local routers and NAS devices, etc. It can be a small-scale PITA to re-enter credentials when cookies expire/get deleted/site has a hiccup. It seems that Firefox’s password manager would be pretty helpful here, but I’ve always shied away from using it.

Assuming that no one has physical access to my machine, do I need to set a highly secure Master Password?

If things go tits up and my machine is compromised by a malicious site (short of a keylogger, which would make this question moot anyway), are all passwords out in the open?

Is it ‘tits up’ or ‘tits-up’?

Malicious sites aside, if someone is probing for weaknesses and open ports (do kids still do that?), could they get to unencrypted passwords? Does having a router-based NAT make that moot?

What about more ‘weighty’ sites like stores or banking?
(I did try to find an answer myself, but Google returned hits from throughout time and space.)