GMFWoodchuck,
Another thought: Have you tried logging into your account from another computer, say, at a friend's house or at the library (if the library doesn't block firearms related sites). Then let a test post sit for 16 minutes on purpose and seeing if it posts?
I was mucking about in security settings not long ago, and find the higher settings will drop cookies after you log off, to avoid accumulating tracking cookies. If you are set up that way, it could be the problem.
The other setting I've run into that matters is choosing whether to let your browser remember passwords or not. Again, for higher security they can be set not to. In Firefox, for example, a window pops up and asks me if I want it to remember my password? For banking or or other critical sites I say "no". But for forums, I say "yes". This way, if my computer is stolen or hacked, that kind of information is annoying but harmless to have loose in the world, while the critical stuff is not.
Another setting that seems to kill my auto log-in in Firefox is deleting the browsing history. Not sure why, but suspect the memorized password is tied to it.