There is a similar extension called "MultiFox" which can be used with
Firefox 4. It is available here:
http://br.mozdev.org/multifox/
Older versions of the extension can also be used with Firefox 3.5.x
and 3.6.x! According to a member of the #Firefox IRC channel (
irc://
irc.mozilla.org/Firefox ), the older versions only separate
"identity profiles" (AKA cookie pools) per window, while the versions
available for Firefox 4 will separate them per tab.
I have not tested the latter functionality as I have yet to install
FF4, but hopefully this extension will be kept up to date and working
until they get the functionality into Firefox's core. If you are
interested, feel free to read this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=117222
I look forward to the day when Firefox implements this functionality
natively.