Is there any way to override HTML's target="_blank" behavior as an end user?

Firefox does allow you to override it:

Users who want to change this behavior need to type in about:config in a tab in the Firefox web browser. This should open the Firefox configuration. First time users need to accept a disclaimer. They then need to filter for the term browser.link.open_newwindow. The default value of that entry is 3 which opens links that would normally open in a new window in a new tab.

To force Firefox to open links (no matter if they have been designed to open in a new tab or window) in the same tab one would need to change the value to [1] which will open all links that would normally open in a new window in the same tab. Changing the value to [2] would open new windows in a new window (duh).

Value of 1: Opens links that would normally open in a new tab or new window in the current tab or window
Value of 2: Open links that would normally open in a new window, in a new window
Value of 3: Open links that would normally open in a new window in a new tab in the current window (default)

If the page has jquery you can inject the following:

$('a[target="_blank"]').removeAttr('target');