[kasabali]

It exists since the very first Firefox version (and probably even before)

After Firefox 3 (which introduced site specific zoom feature) you can set the below config option for old behavior

http://kb.mozillazine.org/Browser.zoom.siteSpecific

[ChrisRR]

I've tried that in the past but it never seems to take effect. I don't know if there's some other config overruling it

Edit: That doesn't look like the config I'm thinking of. That appears to change to zoom for the current active tab, rather than per site. The config I'm thinking of changes zoom for every site