It used to be an option in Firefox. You have to go dig around bugzilla to find the reasons they removed it - https://bugzilla.mozilla.org/show_bug.cgi?id=1249151 https://bugzilla.mozilla.org/show_bug.cgi?id=606655
https://bugzilla.mozilla.org/show_bug.cgi?id=570366#c1
"This option isn't supported, last I checked"
What a reason. They decided to remove it because it "isn't supported"? So much for "open source" being better at "do what users want"... if you personally don't need that option, fine, don't use it. But don't go taking away things that a lot of others want.
I get extremely angry whenever I see discussions like that. You can read and even participate in them, but your opinion is ultimately useless because you're not part of some privileged group who makes all the decisions about what to do with Firefox. It's no better than proprietary software where your bug reports are similarly ignored, besides being possibly a little bit easier to patch.