Can a website's viewport meta tag be over-ridden in Firefox?

I'm hoping there's an add-on to override viewport meta tags. I checked about:config and didn't see anything relevant. A website I use, tumblr, does no browser version checking, and mis-uses the viewport setting to apply a value of "960" to every device. Since my laptop's browser window is 1230 actual pixels, the tumblr webpage is stretched out, which makes images look terrible. Their viewport tag minus the lesser/greater symbols: meta name="viewport" content="width=960" id="viewport" I'd like to override their viewport tag completely, as if it did not exist, or substitute one with <i>content=”width=device-width, initial-scale=1”</i> so I'll have a 1-to-1 pixel ratio. Not much hope of getting tumblr to stop treating all devices the same; their code to get the language version from browsers for font settings has been broken for years. I have reported the viewport problem to tumblr support, however. I'm using Firefox 25.0.1 for Windows (Vista).