I didn't pay too much attention to when this started happening, either before or after a big recent update. On the very bottom left of my firefox window, there's a small box giving me a 404 error. It's out of the way enough to not be a problem, but still really annoying.
empty your cache and clear all cookies. If this does not work, disable your extensions one by one until it disappears.
Removing an adblocker addon and installing a new one fixed this.
For the future, other tips here:
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