Currently IE is the only approved web browser in CIO/OFT. It is against CIO/OFT policy to install different software such as Firefox unless one goes through a cumbersome policy exception application.
In my experience Firefox (with its added extensions) allows more granular control over websites.
Users like me are hesitant to enable active holes like this in their browsers, particularly because with IE it is largely an all-or-nothing enablement: Allow the technology for a Web 2.0 site, and you have allowed it for all the other lousy websites, too.
A strategy to adopt greater Web 2.0 use in agencies would be to allow the use of Firefox and its extensions (e.g. Flash Blocker; No-Script; etc) globally across the board. That way workforce members can "open" the Web 2.0 websites while keeping most websites under tight browser setting control.