And the strange thing is, I have KWallet turned off. Clicking CANCEL dismisses it without it appearing again, the chosen browser launches, and everything is fine. It's merely an annoyance, but I seem to think there may be something off with my KDE (if not the Mint part itself).

Why should this happen, and what can I do about it?

Carver

This may be what you need to do.

The keyring password can be especially annoying when you use automatic login. It's better to disable it. This is how to do that:

a. First delete the current keyrings. Don't be afraid: you won't remove your root password or your user password, but only your personal keyrings. Proceed like this:

Launch a terminal window.
(You can launch a terminal window like this: *Click*)

Use copy/paste to transfer the following command line to the terminal:

rm -v ~/.local/share/keyrings/*.keyring

Press Enter.

b. The next time when you're being prompted for a keyring password, leave the password field blank (simply click Continue and then again Continue, thus agreeing to "unsafe" storage).
Disable the keyring password

My solution was much more simple. I stopped using auto log in. My hard drive is already encrypted and I am the only person who knows the passwords, so for me that was an ok solution.