Author here: I could make this open source if anyone wants to collaborate (would need to tidy up code first, I made this in my noob years and its messy as hell)
Greasemonkey: http://userscripts-mirror.org/scripts/show/136261.html
Bad title, as if everone cares about some facebook specific thing.
Would love a Firefox version!
If anyone is interested in the source for such an extension, I made a similar one and open-sourced it:
https://github.com/typpo/fb-unsee
It's less than 20 lines total!